Use python_bin_path to check for site packages (#12238)

* Use python_bin_path to check for site packages

This fix tries to fix the issue raised in #12232 where
the python invoked the configure.py was used to
check the site packages.

However, the site packages should be checked by the provided
python_bin_path.

This fix fixes the issue. Below is the output after fix:
```
ubuntu@ubuntu:~/tmp/tensorflow$ ./configure
You have bazel 0.5.3 installed.
Please specify the location of python. [Default is /usr/bin/python]: python
Invalid python path: python cannot be found.
Please specify the location of python. [Default is /usr/bin/python]: /usr/bin/python3
Found possible Python library paths:
/usr/local/lib/python3.5/dist-packages
/usr/lib/python3/dist-packages
Please input the desired Python library path to use.  Default is /usr/local/lib/python3.5/dist-packages
Do you wish to build TensorFlow with jemalloc as malloc support? [Y/n]:

...
ubuntu@ubuntu:~/tmp/tensorflow$ ./configure
You have bazel 0.5.3 installed.
Please specify the location of python. [Default is /usr/bin/python]:
Found possible Python library paths:
/usr/local/lib/python2.7/dist-packages
/usr/lib/python2.7/dist-packages
Please input the desired Python library path to use.  Default is /usr/local/lib/python2.7/dist-packages
Do you wish to build TensorFlow with jemalloc as malloc support? [Y/n]:
```

This fix fixes #112232.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Also checks major version through python_bin_path

and use decode for python 3.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>