Exclude *openstack/common* for PEP8 checks and enable E251/E714

It's not necessary to check files in *openstack/common* because
they are imported from oslo and are already checked.

Enabled checks that only had findings in *openstack/common*.

* E251 unexpected spaces around keyword / parameter equals
* E714 test for object identity should be 'is not'

Change-Id: Id7d15b5930804bbb08feb314d6b385c9f4138f42