Restrict subnet create/update to avoid DHCP resync

Restrict subnet create/update to avoid DHCP resync

As we know, IPs in subnet CIDR are used for
1) Broadcast port
2) Gateway port
3) DHCP port if enable_dhcp is True, or update to True
4) Others go into allocation_pools
Above 1) to 3) are created by default, which means if CIDR doesn't
have that much of IPs, subnet create/update will cause a DHCP resync.

This fix is to add some restricts to the issue:
A) When subnet create, if enable_dhcp is True, /31 and /32
   cidrs are forbidden for IPv4 subnets while /127 and /128 cidrs are
   forbidden for IPv6 subnets.
B) When subnet update, if enable_dhcp is changing to True and there are no
   more IPs in allocation_pools, the request should be denied.

Change-Id: I2e4a4d5841b9ad908f02b7d0795cba07596c023d
Co-authored-by: Andrew Boik <dboik@cisco.com>
Closes-Bug: #1443798