Add gate hook scripts for functional tests

Add gate hook to contrib/ci dir to be able to configure functional tests job.

pre_test_hook just imports devstack extension for manila project to the
devstack project.
post_test_hook configures config file for manilaclient based on devstack
installation and runs related tox job.

Also fix default config file path within config module.

Partially implements bp functional-tests

Change-Id: Ibf90af083b9a8e0ec646e1bff676893528ff1914
Merge "Add gate hook scripts for functional tests"