Make BOSH_OPENSTACK_TENANT optional
Make BOSH_OPENSTACK_TENANT optional

[#128305727](https://www.pivotaltracker.com/story/show/128305727)

Signed-off-by: Felix Riegger <9c2aa7530239a330bbac6ac4f56e0946781dfd29@sap.com>

cloudfoundry-incubator/bosh-openstack-cpi-release,cloudfoundry-incubator/bosh-openstack-cpi-release,cloudfoundry-incubator/bosh-openstack-cpi-release