Revert "Allow multiple networks to be created per cluster"

This reverts commit 9530ef323b6b5fdb4dc9ff3516054b19b94c8e60.

The change didn't get enough reviews. There is already reported bug https://bugs.launchpad.net/fuel/+bug/1308041 and there are potential more bugs.
There is also some incosistency in handling admin network. All concerns are described in comments in https://review.openstack.org/#/c/83204/18
There are some concerns about API design addressed in https://review.openstack.org/#/c/83204/16

Change-Id: I2285e52c91692e908008afa377ed26081b4bbd43
Merge "Revert "Allow multiple networks to be created per cluster""