ENH: date_range not working as it intuitively should when specifying start, end, and periods
#### Code Sample, a copy-pastable example if possible

```python
>>> start = pd.Timestamp('2008-01-02 07:51:37.999477')
>>> end = start + pd.Timedelta('2 hours')
>>> pd.date_range(start, end, periods=1000)    # Intuitively a linearly spaced time series
Traceback (most recent call last):

  File "<ipython-input-69-2304a28824c6>", line 1, in <module>
    pd.date_range(start, end, periods=1000)

  File "E:\Anaconda3\lib\site-packages\pandas\core\indexes\datetimes.py", line 2057, in date_range
    closed=closed, **kwargs)

  File "E:\Anaconda3\lib\site-packages\pandas\util\_decorators.py", line 118, in wrapper
    return func(*args, **kwargs)

  File "E:\Anaconda3\lib\site-packages\pandas\core\indexes\datetimes.py", line 324, in __new__
    ambiguous=ambiguous)

  File "E:\Anaconda3\lib\site-packages\pandas\core\indexes\datetimes.py", line 421, in _generate
    raise ValueError('Of the three parameters: start, end, and '

ValueError: Of the three parameters: start, end, and periods, exactly two must be specified
```
#### Problem description

I need a DatetimeIndex object to later use as index in a Series. DatetimeIndex should start at `start`, end at `end` and have a fixed number of elements (1000). Intuitively, this should work with `pd.date_range`, but it doesn't, and I haven't found a good explanation about why this is the case.
I have found a workaround on Stackoverflow (https://stackoverflow.com/questions/25796030/how-can-i-use-pandas-date-range-to-obtain-a-time-series-with-n-specified-perio) that does work:
```python
>>> start = pd.Timestamp('2008-01-02 07:51:37.999477')
>>> end = start + pd.Timedelta('2 hours')
>>> pd.to_datetime(np.linspace(start.value, end.value, 1000))
DatetimeIndex(['2008-01-02 07:51:37.999476992',
               '2008-01-02 07:51:45.206684160',
               '2008-01-02 07:51:52.413891328',
               '2008-01-02 07:51:59.621098496',
               '2008-01-02 07:52:06.828305920',
               '2008-01-02 07:52:14.035513088',
               '2008-01-02 07:52:21.242720256',
               '2008-01-02 07:52:28.449927424',
               '2008-01-02 07:52:35.657134592',
               '2008-01-02 07:52:42.864341760',
               ...
               '2008-01-02 09:50:33.134612224',
               '2008-01-02 09:50:40.341819392',
               '2008-01-02 09:50:47.549026560',
               '2008-01-02 09:50:54.756233728',
               '2008-01-02 09:51:01.963440896',
               '2008-01-02 09:51:09.170648064',
               '2008-01-02 09:51:16.377855488',
               '2008-01-02 09:51:23.585062656',
               '2008-01-02 09:51:30.792269824',
               '2008-01-02 09:51:37.999476992'],
              dtype='datetime64[ns]', length=1000, freq=None)
```

#### Expected Output
```python
>>> start = pd.Timestamp('2008-01-02 07:51:37.999477')
>>> end = start + pd.Timedelta('2 hours')
>>> pd.date_range(start, end, periods=1000)
DatetimeIndex(['2008-01-02 07:51:37.999476992',
               '2008-01-02 07:51:45.206684160',
               '2008-01-02 07:51:52.413891328',
               '2008-01-02 07:51:59.621098496',
               '2008-01-02 07:52:06.828305920',
               '2008-01-02 07:52:14.035513088',
               '2008-01-02 07:52:21.242720256',
               '2008-01-02 07:52:28.449927424',
               '2008-01-02 07:52:35.657134592',
               '2008-01-02 07:52:42.864341760',
               ...
               '2008-01-02 09:50:33.134612224',
               '2008-01-02 09:50:40.341819392',
               '2008-01-02 09:50:47.549026560',
               '2008-01-02 09:50:54.756233728',
               '2008-01-02 09:51:01.963440896',
               '2008-01-02 09:51:09.170648064',
               '2008-01-02 09:51:16.377855488',
               '2008-01-02 09:51:23.585062656',
               '2008-01-02 09:51:30.792269824',
               '2008-01-02 09:51:37.999476992'],
              dtype='datetime64[ns]', length=1000, freq=None)
```

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 44 Stepping 2, GenuineIntel
byteorder: little
LC_ALL: None
LANG: en
LOCALE: None.None

pandas: 0.22.0
pytest: 3.3.2
pip: 9.0.1
setuptools: 38.4.0
Cython: 0.27.3
numpy: 1.14.2
scipy: 1.0.1
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.6.6
patsy: 0.5.0
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.4
feather: None
matplotlib: 2.1.2
openpyxl: 2.4.10
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.1
bs4: 4.6.0
html5lib: 0.9999999
sqlalchemy: 1.2.1
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
