openstack: add --test-repo CLI option

openstack: add --test-repo CLI option

Add custom repos before installing rpm packages on test nodes.
Repository can be specified as a NAME:URL pair. Several repos
can be provided by specifying the option multiple times.
For example,
    --test-repo foo:http://example.com/repo/foo \
    --test-repo bar:http://example.com/repo/bar

gives two test package repositories named "foo" and "bar".

Conflicts:
	scripts/openstack.py
	teuthology/openstack/__init__.py
	teuthology/orchestra/opsys.py
	teuthology/task/install/rpm.py