Add support for tenant_id based authentication with Neutron

Keystone v3 supports non-unique project/tenant names, so
Nova should switch to using tenant ID for admin authentication.

Implements blueprint tenant-id-based-auth-for-neutron
DocImpact: Adds new flag, nova_admin_tenant_id, and deprecates
existing flag, nova_admin_tenant_name

Change-Id: I4a4ffe84fdcf98ace81fd148f096cad483aad96c
Merge "Add support for tenant_id based authentication with Neutron"