Add ability to configure tempest tests based on network extensions

This patch introduces a Devstack variable to specify which network
extensions are enabled; this is useful for configuring tempest runs
when third-party plugins that do not support certain extensions.

Closes-bug: #1247778
Closes-bug: #1231152

Change-Id: Iee170993cb164502774f9ac4201b963d9a2715ba
Merge "Add ability to configure tempest tests based on network extensions"