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
Do not assume order of network_uuid's

This fixes the neutron.tests.unit.test_linux_dhcp.TestDnsmasq
.test_existing_dhcp_networks unit test that breaks with a
randomized PYTHONHASHSEED (see the bug report).

The test assumed that the network_uuid's list was sorted, so sort
the result before comparing.

Note: There are several other unrelated unit tests that also break with a
randomized PYTHONHASHSEED, but they are not addressed here. They will be
addressed in separate patches.

Partial-bug: #1348818
Change-Id: Ia6f742e1fae0687164b841beebb4efcf31ff2625
Merge "Make sure that gateway is in CIDR range by default"
Merge "Do not assume order of network_uuid's"