Now the label `fixed_network_cidr` is not handled correctly, no matter
if the label is set, the default value '10.0.0.0/24' is used for
fixed network anyway. This patch fixes it and renamed it as
`fixed_subnet_cidr` to make less confusion. The new behaviour will be:
1. If the label `fixed_subnet_cidr` is set but no fixed subnet passed
in, then a new subnet will be created with the given CIDR.
2. If a fixed subnet is passed in by user, then label `fixed_subnet_cidr`
will be override with the CIDR from the given subnet.
Task: 39847
Story: 2007712
Change-Id: Id05e36696bf85297a556fcd959ed897fe47b7354