Stop device_owner from being set to 'network:*'

Stop device_owner from being set to 'network:*'

This patch adjusts the FieldCheck class in the policy engine to
allow a regex rule. It then leverages that to prevent users from
setting the device_owner field to anything that starts with
'network:' on networks which they do not own.

This policy adjustment is necessary because any ports with a
device_owner that starts with 'network:' will not have any security
group rules applied because it is assumed they are trusted network
devices (e.g. router ports, DHCP ports, etc). These security rules
include the anti-spoofing protection for DHCP, IPv6 ICMP messages,
and IP headers.

Without this policy adjustment, tenants can abuse this trust when
connected to a shared network with other tenants by setting their
VM port's device_owner field to 'network:<anything>' and hijack other
tenants' traffic via DHCP spoofing or MAC/IP spoofing.

Closes-Bug: #1489111
Change-Id: Ia64cf16142e0e4be44b5b0ed72c8e00792d770f9