Added PyRSS2Gen and python-swiftclient to pip-requires to satisfy the openstackwatch dependency.

Change-Id: I2780ff3f1392a049342895cd3b07edd5d8e5bff7
Reviewed-on: https://review.openstack.org/24605
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins