`RollingGroupBy.quantile` ignores `interpolation` argument
#### Code Sample, a copy-pastable example if possible

```python
df = pd.DataFrame([
    ['a', 1], ['a', 2], ['a', 3],
    ['b', 1], ['b', 3], ['b', 5]
], columns=['key', 'val'])
print(df.groupby('key').rolling(2).quantile(.4, interpolation='lower'))
```
#### Problem description

The keyword argument `interpolation` is ignored - no matter what value it is set to, the default (linear) interpolation is used.  This problem only applies to `RollingGroupBy.quantile` - `GroupBy.quantile` and `Rolling.quantile` both appear to work as expected.

#### Expected Output

```
       val
key       
a   0  NaN
    1  1.0
    2  2.0
b   3  NaN
    4  1.0
    5  3.0
```

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

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.4.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 17.7.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 0.25.1
numpy            : 1.17.2
pytz             : 2019.2
dateutil         : 2.8.0
pip              : 19.2.3
setuptools       : 41.2.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.10.1
IPython          : 7.8.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None