Include extras/ dir in flake8 source code check

Moving forward all python scripts in extras/ are to
be checked for pep8 compliancy.

Change-Id: I4f20c1c97590049ba59fd432cbabba89df101e81
Signed-off-by: Prashanth Pai <ppai@redhat.com>
Reviewed-on: http://review.gluster.org/5714
Reviewed-by: Peter Portante <pportant@redhat.com>
Reviewed-by: Luis Pabon <lpabon@redhat.com>
Tested-by: Luis Pabon <lpabon@redhat.com>