Use renamed _fail_second_call() in cisco nexus tests

In commit 5e4b0c6fc6670ea036d801ce53444272bc311929 NeutronDbPluginV2TestCase
_do_side_effect() was renamed to _fail_second_call(), but the nexus test cases
were not updated. The tests still worked because they check for
HTTPInternalServerError which is the end result of the server encountering
"AttributeError: object has no attribute '_do_side_effect'". So the fault
injection still worked, but not in the way intended.

Note: the monolithic nexus plugin is not updated here since it is being
removed from the tree.

Closes-bug: #1367552

Change-Id: I083285fff5144d8dc35bd5b64faa7e400c68bcff
Remove useless check in _rpc_update_firewall

Firewall_db_mixin.update_firewall returns an updated firewall or raises
an error (if the firewall is not found), that's why checking if its
result is evaluated to False is not required

Change-Id: Ib4e2fa0920783728b8e1cc8597b3edf78e098b52
Passing admin tenant name to EOS

The Arista ML2 plugin was not passing the admin tenant name to EOS without which
it is not possible to authenticate with keystone using just the admin name
and password. This patch passes the admin tenant name along with the admin
credentials.

Change-Id: I6c8b872087d17da2c3de43186d1916fc368dd786
Closes-Bug: 1359417
Merge "Use renamed _fail_second_call() in cisco nexus tests"
Merge "Passing admin tenant name to EOS"
Merge "Remove useless check in _rpc_update_firewall"