Refactoring code that gives possibility define admin tenant name for Neutron.

In this commit removed unnecessary parameters.
Tenant's name gets from Neutron config if given there, from fuel_config hash
if not given in Neutron config and set to "admin" if not given anywhere.

Closes-bug: #1284189
Change-Id: I9476e6fb81552a9b58d8573c2f587fde417871cd