Replace neutron-grenade job with grenade-py3
Replace neutron-grenade job with grenade-py3

Since the grenade-py3 job is now in the neutron check and gate queues,
the neutron team would like to eliminate the neutron-grenade job as it
look like it duplicates the functionality of grenade-py3. This also
means the grenade job is now py3-compliant.

Change-Id: I83e359974630e2d955bb93574d460b5a136927df
Needed-By: https://review.openstack.org/620357

openstack/python-openstacksdk,dtroyer/python-openstacksdk,dtroyer/python-openstacksdk,openstack/python-openstacksdk,stackforge/python-openstacksdk,stackforge/python-openstacksdk