Add api extension for new network fields.

This uses the existing api extension to implement the actual control
of the fields, but the check is based on a new dummy extension
called os-extended-networks.

Api sample tests added for new extension.

DocImpact: Adds an extension that enables extra fields for network
create. The new fields are:
  mtu: int (default flag) if set, nova sets the mtu on bridge. This
       allows network_device_mtu flag to be set per network.
  dhcp_server: ip (default == gateway) if different from gateway, sets
                nova to assume gateway is external.
  enable_dhcp: bool (default true) false will disable dhcp on network.
  share_address: bool (default flag) if specifed, network will have
                 the same dhcp ip on every host. This allows
                 share_dhcp_address flag to be set per network.
  allowed_start: ip if specified, reserves all ips before allowed_start.
  allowed_end: ip if specified, reserves all ips after allowed_end.

Partially-implements blueprint better-support-for-multiple-networks

Change-Id: I577fe5f6560be50106f345a42a826e97d5e7d64c
Merge "Add api extension for new network fields."