Constructing a Period having a multiple of the freq ignores the freq multiplier
When using a multiple of the freq, the `period_range` function behaves as expected - e.g. for a  half-yearly frequency (with a `DEC` anchor) it constructs a `PeriodIndex` with periods starting on the 1st and 3rd quarters:
```python
In [27]: pd.period_range('01-Jan-2017', '01-Jan-2019', freq='2Q-DEC')[:-1]
Out[27]: PeriodIndex(['2017Q1', '2017Q3', '2018Q1', '2018Q3'], dtype='period[2Q-DEC]', freq='2Q-DEC')
```
I would expect that when constructing a `Period` with a freq of `2Q-DEC` that you should only be able to get periods starting on the 1st and 3rd quarters - i.e. any date in[Q1, Q3) would give a period starting on Q1 and any date in [Q3, Q1) would give a period starting on Q3. What currently happens is that it appears to ignore the frequency multiplier and treat the periods if they were simply straight quarters:
```python
In [31]: [pd.Period('01-Feb-2017', freq='2Q'),
    ...: pd.Period('01-May-2017', freq='2Q'),
    ...: pd.Period('01-Aug-2017', freq='2Q'),
    ...: pd.Period('01-Nov-2017', freq='2Q')]
Out[31]: 
[Period('2017Q1', '2Q-DEC'),
 Period('2017Q2', '2Q-DEC'),
 Period('2017Q3', '2Q-DEC'),
 Period('2017Q4', '2Q-DEC')]
```
i.e. I would expect instead:
```python
[Period('2017Q1', '2Q-DEC'),
 Period('2017Q1', '2Q-DEC'),
 Period('2017Q3', '2Q-DEC'),
 Period('2017Q3', '2Q-DEC')]
```