Align some defaults with with ControllerCount default

Align some defaults with with ControllerCount default

NeutronDhcpAgentsPerNetwork is normally correlated with the
number of controllers, so given that the template
defaults the ControllerCount to 1, 1 seems like a more consistent
value for NeutronDhcpAgentsPerNetwork, and it's consistent with
what we test in CI.

Also NeutronL3HA defaults to False, which is consistent with the default
ControllerCount of 1, but NeutronAllowL3AgentFailover defaults to
True, which I assume makes no sense if there's only 1 controller
and no HA

Change-Id: I4612060979d1e4381800f30e359f58aeef61b861