With this fix 'force_gateway_on_subnet' configuration
option is removed, and gateway outside the subnet is
always allowed. Gateway cannot be forced onto to the
subnet range.
DocImpact: All references of 'force_gateway_on_subnet'
configuration option and its description should be
removed from the docs.
Change-Id: I1a676f35828e46fcedf339235ef7be388341f91e
Closes-Bug: #1548193