engine: Remove "floating network" support in add

Removed support of so called "floating network" from AddNetworkCommand
since this state is not currently supported in the system, and either
way clients always send a DC ID in the parameters.

Change-Id: Icb323fd274f0991bc0d7d4d831f65925ff078978
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>