customization: changed bridge setup customization

Avoid to ask about nics if the bridge is already created.

At bridge customization provide a list of valid nics.
Ensure that the default nic exists.

Removed todo about asking bridge name: it should not be
changed and if really needed it can be changed through
environment variable.

Change-Id: I3c63c902f2a7e34b784810917380a537f51ddea5
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>