sort_index(..., ascending=False, ...) overrides sort_remaining behavior with multiindex
#### Script to reproduce

```python
import pandas as pd
import numpy as np

df = pd.DataFrame(np.random.rand(6))
df['index_1'] = [3, 3, 2, 2, 1, 1]
df['index_2'] = [2, 1, 2, 1, 2, 1]
df = df.set_index(['index_1', 'index_2'])

print(df)
print(df.sort_index(level='index_1', sort_remaining=False))
print(df.sort_index(level='index_1', ascending=False, sort_remaining=False))
```

#### iPython example output

```python

In [1]: import pandas as pd

In [2]: import numpy as np

In [3]: df = pd.DataFrame(np.random.rand(6))

In [4]: df['index_1'] = [3, 3, 2, 2, 1, 1]

In [5]: df['index_2'] = [2, 1, 2, 1, 2, 1]

In [6]: df = df.set_index(['index_1', 'index_2'])

In [7]: df
Out[7]:
                        0
index_1 index_2
3       2        0.558019
        1        0.096064
2       2        0.353176
        1        0.153776
1       2        0.812181
        1        0.313342

In [8]: df.sort_index(level='index_1', sort_remaining=False)
Out[8]:
                        0
index_1 index_2
1       2        0.812181
        1        0.313342
2       2        0.353176
        1        0.153776
3       2        0.558019
        1        0.096064

In [9]: df.sort_index(level='index_1', ascending=False, sort_remaining=False)
Out[9]:
                        0
index_1 index_2
3       1        0.096064
        2        0.558019
2       1        0.153776
        2        0.353176
1       1        0.313342
        2        0.812181
```

#### Problem description

Documentation for `sort_index()`: https://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.sort_index.html

The `sort_index()` method will sort the dataframe by the index values. If the dataframe has a multiindex, then it sorts by the indexes in order. However, if you only want to sort by a subset of the indexes and leave the others in their current order, you can use the `level` keyword argument to specify a level (or list of levels) to sort on, and set `sort_remaining=False` to ignore the other levels. The `sort_remaining` keyword seems to work correctly when `ascending=True` (default), but when passing `ascending=False` and `sort_remaining=False`, Pandas continues to sort the other indices (in fact in ascending order, too).

Note that this happens regardless if you specify levels in any of the supported formats; viz.

```python
df.sort_index(level=0, ascending=False, sort_remaining=False)
df.sort_index(level=[0], ascending=False, sort_remaining=False)
df.sort_index(level='index_1', ascending=False, sort_remaining=False)
df.sort_index(level=['index_1'], ascending=False, sort_remaining=False)
```

There have been a few issues/PRs on the `sort_index()` method in the past year, so I'm not sure if one of those PRs broke this ability or whether this has been around longer or is unrelated.

#### Expected Output

```python
In [9]: df.sort_index(level='index_1', ascending=False, sort_remaining=False)
Out[9]:
                        0
index_1 index_2
3       2        0.558019
        1        0.096064
2       2        0.353176
        1        0.153776
1       2        0.812181
        1        0.313342
```

Note that the expected outcome could be achieved in this example with 

```python
df.reset_index('index_2').sort_index(ascending=False).reset_index().set_index(['index_1', 'index_2'])
```

or similar setting/resetting of indexes. Well, or you could just do nothing since the dataframe is already in that order, but you get the point.

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.2.final.0
python-bits: 64
OS: Darwin
OS-release: 18.0.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.23.4
pytest: None
pip: 18.0
setuptools: 36.4.0
Cython: None
numpy: 1.13.1
scipy: 0.19.1
pyarrow: None
xarray: None
IPython: 6.1.0
sphinx: 1.7.4
patsy: None
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.1.1
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>