Configurable fixed network name

The fixed network gets his name when you use the:
nova-manage network create "$PRIVATE_NETWORK_NAME" $FIXED_RANGE 1 $FIXED_NETWORK_SIZE $NETWORK_CREATE_ARGS
command.

It can be different in every deployment, so it must be configurable.

Change-Id: I5c724565561b1a230372b8fc1693cb81fea3117e
Merge "Configurable fixed network name"