to_datetime uses previous row's timezone when timezone not specified and utc=True
#### Code Sample, a copy-pastable example if possible

Without utc=True:
```python
pd.to_datetime([
    '2018-11-28T00:00:00', 
    '2018-11-28T00:00:00+12:00', 
    '2018-11-28T00:00:00', 
    '2018-11-28T00:00:00+06:00', 
    '2018-11-28T00:00:00'
])
```

Result:
```
Index([      2018-11-28 00:00:00, 2018-11-28 00:00:00+12:00,
             2018-11-28 00:00:00, 2018-11-28 00:00:00+06:00,
             2018-11-28 00:00:00],
      dtype='object')
```

With utc=True:
```python
pd.to_datetime([
    '2018-11-28T00:00:00', 
    '2018-11-28T00:00:00+12:00', 
    '2018-11-28T00:00:00', 
    '2018-11-28T00:00:00+06:00', 
    '2018-11-28T00:00:00'
], utc=True)
```

Result:
```
DatetimeIndex(['2018-11-28 00:00:00+00:00', '2018-11-27 12:00:00+00:00',
               '2018-11-27 12:00:00+00:00', '2018-11-27 18:00:00+00:00',
               '2018-11-27 18:00:00+00:00'],
              dtype='datetime64[ns, UTC]', freq=None)
```

#### Problem description

The behavior leads to unexpected and inconsistent treatment of datetimes where the timezone is not explicitly specified. Simply using the last timezone it saw from a datetime where the timezone was specified is pretty confusing.

#### Expected Output
I would be fine with it just assuming that any datetimes where the timezone is NOT specified are already in UTC. Another valid way of handling it would be to use os.environ['TZ']. Another valid way would be to add another input argument to to_datetime that allowed specifying which timezone to use when there is no timezone specified. Any of these would lead to conclusive, explainable results. This is what I would like to see:

```
DatetimeIndex(['2018-11-28 00:00:00+00:00', '2018-11-27 12:00:00+00:00',
               '2018-11-28 00:00:00+00:00', '2018-11-27 18:00:00+00:00',
               '2018-11-28 00:00:00+00:00'],
              dtype='datetime64[ns, UTC]', freq=None)
```

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.7.2.final.0
python-bits: 64
OS: Darwin
OS-release: 18.2.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.24.0
pytest: None
pip: 19.0.1
setuptools: 40.7.0
Cython: None
numpy: 1.16.0
scipy: 1.2.0
pyarrow: None
xarray: None
IPython: 7.2.0
sphinx: None
patsy: None
dateutil: 2.7.5
pytz: 2018.9
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 3.0.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml.etree: 4.3.0
bs4: None
html5lib: None
sqlalchemy: 1.2.17
pymysql: None
psycopg2: 2.7.7 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

</details>
