Unstacking a MultiIndex with integer names is ambiguous
#### Code Sample, a copy-pastable example if possible

```python
In [19]: index = pd.MultiIndex.from_tuples([('one', 'a'), ('one', 'b'),
    ...: ...                                    ('two', 'a'), ('two', 'b'
    ...: )])

In [20]: s = pd.Series(np.arange(1.0, 5.0), index=index)

In [21]: s
Out[21]:
one  a    1.0
     b    2.0
two  a    3.0
     b    4.0
dtype: float64

In [22]: s.unstack(0)
Out[22]:
   one  two
a  1.0  3.0
b  2.0  4.0

In [24]: s.index.names= [1,0]

In [25]: s.unstack(0)
Out[25]:
0      a    b
1
one  1.0  2.0
two  3.0  4.0
```
#### Problem description

Related #14969

When calling `unstack(level=1)` on a `MultiIndex` with integer names,  it is ambiguous whether to unstack by level name or the level position since both are valid level identifiers in unstack() (but will prioritize level name)  

While possibly an uncommon case, a keyword identifier in `unstack()` may help clarify whether the user wants to unstack by the level position or level name.

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

<details>

```
INSTALLED VERSIONS
------------------
commit: 465c59f964c8d71d8bedd16fcaa00e4328177cb1
python: 2.7.12.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-45-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: None.None

pandas: 0.21.0.dev+313.g465c59f
pytest: 3.0.6
pip: 8.1.2
setuptools: 27.2.0
Cython: 0.24.1
numpy: 1.11.2
scipy: None
pyarrow: None
xarray: None
IPython: 5.1.0
sphinx: 1.4.8
patsy: None
dateutil: 2.5.3
pytz: 2016.7
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 1.5.3
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
s3fs: None
pandas_gbq: None
pandas_datareader: None
```
</details>
