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 "Passing admin tenant name to EOS"