Enable network to be scheduled to N DHCP agents

Fixes bug 1174132

Change-Id: Iabe96fd8a98f3539eb21268fb5ef58df8dbd8782
Reference OVS OptGoup names in lowercase

To have consistent option group name format across projects,
oslo.config now normalizes all non lowercase group names to lowercase
when loading conf files.
Thus conf files are backwards compatible but option group references
in the code must now use lowercase before we update oslo.config.

This patch replaces all 'OVS' references to 'ovs'

Change-Id: Id8697138d822e331a6776f847915c61b40f8dd6a
Restore correct alias for network-gateway extension

Bug 1182682

Change-Id: I85a93e6ccac32515c7ebb2dfe41ba67cf11c9d9f
Merge "Enable network to be scheduled to N DHCP agents"
Merge "Restore correct alias for network-gateway extension"