Implements the missing configuration options and context to configure
allow-automatic-l3agent-failover and allow-automatic-dhcp-failover
options in neutron.conf. The options are respectively added to
versions juno and kilo upwards as those are the Neutron versions
since they were first supported.
Change-Id: I7e465b7fef13f61f7b37135a86ac2f590c0c9be6
Closes-Bug: #1799956
5f255bc9f9