MultiIndex slicing with partial string match of datetimes fails
Multilevel indexing doesn't work if IndexSlice with a range is provided.


#### Code Sample

```python

import pandas as pd
from itertools import cycle
dt_index = pd.date_range('1999-07', '1999-11')
sec_index = ['black', 'red', 'white']
items = []
for d, c in zip(dt_index, cycle(sec_index)):
    items.append({
     'date': d,
        'kind': c,
        'content': '{} - {}'.format(d, c)
    })
df = pd.DataFrame(items).set_index(['date', 'kind'], drop=False)
print(len(df))

# it works when indexing matches single item in index
single_item_index = df.loc[pd.IndexSlice[:'1999-09-02', :], :]
print(len(single_item_index))

# but doesn't work when slice has a range of items
multi_items_index = df.loc[pd.IndexSlice[:'1999-09', :], :]
```

#### Problem description

Code above raises

```
.../python3.6/site-packages/pandas/core/indexes/multi.py in _get_level_indexer(self, key, level, indexer)
   2635                 # note that the stop ALREADY includes the stopped point (if
   2636                 # it was a string sliced)
-> 2637                 return convert_indexer(start.start, stop.stop, step)
   2638 
   2639             elif level > 0 or self.lexsort_depth == 0 or step is not None:

AttributeError: 'int' object has no attribute 'start'
```

As seen in the code, providing slice `pd.IndexSlice[:'1999-09', :]` with  range as a 'stop' fails. 

Problems seems arise from OR logic in `if isinstance(start, slice) or isinstance(stop, slice):` of 'core/indexes/multi.py#_get_level_indexer', when `start` defaults to 0 or points to particular element.


#### Expected Output

Expect to have slice of the dataframe in range `[:'1999-09', :]`, same way as it works for single-level index

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.7.final.0
python-bits: 64
OS: Linux
OS-release: 4.18.10-arch1-1-ARCH
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.24.0
pytest: None
pip: 18.0
setuptools: 39.2.0
Cython: 0.28.5
numpy: 1.14.3
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 6.4.0
sphinx: None
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: 1.2.1
tables: 3.4.4
numexpr: 2.6.6
feather: None
matplotlib: 2.2.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml.etree: 4.2.1
bs4: 4.6.0
html5lib: 0.9999999
sqlalchemy: 1.2.8
pymysql: None
psycopg2: 2.7.4 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

</details>
