Run flake8 also on cinder/common

Run flake8 also on cinder/common

Even though testenvs pep8 and pep8-constraints included cinder/common
directory to be processed by flake8, global configuration in flake8
secion was excluding common glob preventing the directory from being
checked.

This patch removes glob from flake8 exclude section, unnecessary
cinder/common inclusion when running flake8, and fixes
cinder/common/config.py.

If we had a common directory in our code root and wanted to exclude it,
glob should be ./common instead of just common like we had now.

Change-Id: Iefc64c02335ed9296bda323fdcb5688125fb4e77