Currently we define fixed_network_name only when
tempest_create_isolated_networks == False.
It makes things complicated because tempest_create_isolated_networks is
applicable only when dynamic credentials are used.
fixed_network_name should be also defined when dynamic credentials
are not used.
Basically the only scenario when fixed_network_name should not be
defined is using dynamic credentials together with
tempest_create_isolated_networks.
fixed_network_name should be defined in all other cases.
Change-Id: I547d70995de5afbdc84c9f4e86b2599ad4cb100b