Tempest: configure exact set of extensions to test

So far devstack configures tempest either for testing all extensions
or a specific subset. It does not allow users for specifying a set
of extensions which should not be exercised.

This patch adds this support. To this aim, the tempest configuration
process will scan API endpoints for active extensions using the
verify_tempest_config.py tool, and then will remove those extensions
which have been explicitly disabled by the user.

If an explicit subset of extensions to enable is passed to devstack,
tempest will use this subset, rather than the list of active
extensions.

Implements blueprint branchless-tempest-extensions

Change-Id: I263bcf04668953f414a4ef18cb98c1c373e142ad
Merge "Tempest: configure exact set of extensions to test"
Merge commit 'refs/changes/34/140634/1' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch