xenapi: enable user to specify FLAT_NETWORK_BRIDGE

install_os_domU.sh failed, if the FLAT_NETWORK_BRIDGE is found in
localrc. As nova looks up the network by either name-label or bridge
name, it makes sense to enable the user to specify this parameter. As an
example, if the user wants to use name-labels to specify networks, and
those name-labels could be used in domU to create bridges:

VM_BRIDGE_OR_NET_NAME="osvmnet"
FLAT_NETWORK_BRIDGE="osvmnet"

In this case, the domU will know only about a name label, so it could be
decoupled from which xapi bridges used.

This change also adds some fixes (missing double quotes).

Change-Id: I045e367ef441be20c4e8cb8af3c1149392db796b
Merge "xenapi: enable user to specify FLAT_NETWORK_BRIDGE"