API/DEPR: 'periods' argument instead of 'n' for DatetimeIndex.shift() 
- [x] closes #22458 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

In order to be consistent with `Index.shift` & `Series.shift`, `n` argument was deprecated in favor of `periods`.

```
In [2]: idx = pd.DatetimeIndex(start='2014-08-01 10:00', freq='H',
   ...:                         periods=3, tz='Asia/Calcutta')
   ...:                         

In [3]: idx
Out[3]: 
DatetimeIndex(['2014-08-01 10:00:00+05:30', '2014-08-01 11:00:00+05:30',
               '2014-08-01 12:00:00+05:30'],
              dtype='datetime64[ns, Asia/Calcutta]', freq='H')

In [4]: idx.shift(1, freq=None)
Out[4]: 
DatetimeIndex(['2014-08-01 11:00:00+05:30', '2014-08-01 12:00:00+05:30',
               '2014-08-01 13:00:00+05:30'],
              dtype='datetime64[ns, Asia/Calcutta]', freq='H')

In [5]: idx.shift(periods=1, freq=None)
Out[5]: 
DatetimeIndex(['2014-08-01 11:00:00+05:30', '2014-08-01 12:00:00+05:30',
               '2014-08-01 13:00:00+05:30'],
              dtype='datetime64[ns, Asia/Calcutta]', freq='H')

In [6]: idx.shift(n=1, freq=None)
//anaconda/envs/pandas_dev/bin/ipython:1: FutureWarning: the 'n' keyword is deprecated, use 'periods' instead
  #!//anaconda/envs/pandas_dev/bin/python
Out[6]: 
DatetimeIndex(['2014-08-01 11:00:00+05:30', '2014-08-01 12:00:00+05:30',
               '2014-08-01 13:00:00+05:30'],
              dtype='datetime64[ns, Asia/Calcutta]', freq='H')
```