Ensure flake8 tests run on all api code

We need to exclude the plugins/xenserver directory
because the python2.4 code can't use as in except clauses.
The existing exclude directory accidentally included some
code in nova/api/openstack/compute/plugins and the related
tests. This code is now included in the flake8 tests,
and the current violations have been fixed up.

Fixes bug 1197858

Change-Id: Ib5f1b65d5d793f0f5854e898ca3ce751e43ef233
Merge "Ensure flake8 tests run on all api code"