Make sure that gateway is in CIDR range by default

Git commit c3706fa2 introduced the force_gateway_on_subnet
option that verified that the defined gateway is in the CIDR
range of a newly created or updated subnet. However, the default
value was False for backwards compatability reasons. The default
will change to True and the option will be marked as deprecated.

For IPv6, the gateway must be in the CIDR only if the gateway
is not a link local address.

DocImpact
Change-Id: I04fd1caec6da5dceee3f736b3f91f2468150ba2a
Closes-Bug: #1304181
Merge "Make sure that gateway is in CIDR range by default"