This CR allows running "functional" tox job for different backends.
Short description about expected workflow:
1. Gate job run ceilometer/ceilometer/tests/functional/hooks/post_test_hook.py
with <backend> parameter.
2. This script run "tox -efunctional" command with defined variable
CEILOMETER_TEST_BACKEND
3. After in "run-functional-tests.sh" setup script for the backend
and testr are runned.
In this CR running testr with pretty_tox.sh script added because
it allows to use subunit-trace output which developed in tempest-lib
and improve useful of testr output.
Partially implements: blueprint ceilometer-functional-tests
Change-Id: Idb66aca0b46779516db2baec856df8223dbe5c13