new behavior regarding `Series.plot.bar` for `logy=True` in 0.21
#### Code Sample
```python
Series([10**x for x in range(5)]).plot.bar()
Series([10**x for x in range(5)]).plot.bar(logy=True)
```
#### Problem description

The behavior changed from last version, 0.20. In last version, `Series.plot.bar` uses one color for bars. In the new version, it uses different colors for bars even for a single series. When using `logy` argument, the bars cannot be seen. It seems the fill color is same as the background color.

![image](https://user-images.githubusercontent.com/1349042/33044244-31e3451e-ce16-11e7-9c2e-ca86ac0664ec.png)
![image](https://user-images.githubusercontent.com/1349042/33044250-363cfc68-ce16-11e7-9dec-1b115e399369.png)

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.21.0
pytest: 3.2.1
pip: 9.0.1
setuptools: 36.5.0.post20170921
Cython: 0.26.1
numpy: 1.13.3
scipy: 1.0.0
pyarrow: 0.7.0
xarray: None
IPython: 6.1.0
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.2
feather: 0.4.0
matplotlib: 2.1.0
openpyxl: 2.4.8
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.0
bs4: 4.6.0
html5lib: 0.999999999
sqlalchemy: 1.1.13
pymysql: 0.7.11.None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: 0.5.0]

</details>
