devstack should put admin_tenant_id in tempest.conf file while setup

Some of the test cases in tempest related to quota use
admin_tenant_name, ideally it should use admin_tenant_id.
As tempest.conf file does not contains admin_tenant_id
it should also be updated. tempest.conf file is generated
by devstack while setup of openstack, changes should be made
in devstack so as to put admin_tenant_id in tempest conf file.

Change-Id: I62baf5b1e21ebf8d3b511624c54741015b304955
Closes-Bug: #1321624
Merge "devstack should put admin_tenant_id in tempest.conf file while setup"