tests.py files are excluded from targets checked by pylint
because it takes longer and longer time to check them somehow.

pylint 2.2.x is supported only in Python 3 and we already switched
our pep8 basepython to python3, so pylint and astroid are installed
only for python3. This seems the easiest way to ensure py27 env works.

literal-comparison error in openstack_dashboard/api/nova.py
is also fixed in this commit as it is simple enough.

Change-Id: Ic8868a44b296dba457be721716ca6f9d37ad9369