Remove useless requirements
Remove useless requirements

This patch cleans up the requirements.txt list to remove
greenlet, httplib2 and iso8601 modules not used anymore.

Change-Id: Ie51b031f8b83960e6718faa36a86d7a33f721e64

zeinsteinz/tacker,openstack/tacker,zeinsteinz/tacker,stackforge/tacker,openstack/tacker,openstack/tacker,stackforge/tacker