DataFrame.plot doesn't handle axis titles for sharex=row correctly
#### Code Sample, a copy-pastable example if possible

```python
import matplotlib.pyplot as plt
import pandas as pd

_, axs = plt.subplots(2, 3, sharex='row')

for ax in axs.flatten():
    ax.plot(range(5))
    ax.set_xlabel('x-axis title')

ax = axs[1, 0]
data = [2]*5
pd.DataFrame(data).plot(ax=ax) # removes xaxis and yaxis labels except those on the sides of the grid.

plt.tight_layout() # so xaxis label can't hide beneath second plot

```
#### Problem description

When sharing axes on subplots only by row or column, the Pandas Dataframe plotting method eliminates all axis titles that are not on the far left or bottom right. This gets especially confusing since doing just one Pandas plot screws up the entire subplot grid. In contrast, the Matplotlib (`Axes.plot`) and Pyplot (`plt.plot`) plotting methods don't try to be smart with axis labels at all and leave them where they are.

The problem occurs for all four combinations of `sharex`, `sharey`, `'row'` and `'col'`.

A workaround is to iterate through the axes and set `ax.xaxis.label.set_visible(True)` which has to be done after the last Pandas operation on any Axes contained in the Figure.

#### Expected Output

Three options:

1. Do not modify axis labels to maintain consistency with `matplotlib.axes.Axes.plot()` and `matplotlib.pyplot.plot()`. I. e. keep axis labels on all plots.

2. Only remove x-axis labels on axes shared with an axes beneath. Only remove y-axis labels on axes shared with an axes to the left.
a) Do this for all axes in the subplot.
b) Only modify axis labels on axes that pandas actually plots to. (Sounds like a nightmare to implement.)
c) Include a keyword argument in `pandas.Dataframe.plot` that allows the user to control whether or not axis labels are modified.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.14.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 45 Stepping 7, GenuineIntel
byteorder: little
LC_ALL: None
LANG: en
LOCALE: None.None

pandas: 0.23.0
pytest: 3.5.0
pip: 9.0.3
setuptools: 39.0.1
Cython: 0.28.2
numpy: 1.14.2
scipy: 1.0.1
pyarrow: None
xarray: None
IPython: 5.6.0
sphinx: 1.7.2
patsy: 0.5.0
dateutil: 2.7.2
pytz: 2018.4
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.4
feather: None
matplotlib: 2.2.2
openpyxl: 2.5.2
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.4
lxml: 4.2.1
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.6
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
