This is currently calculated in t-h-t but has a hard-coded reference
to the ControllerCount which is incompatible with custom-roles.
So instead calculate the setting based on the number of neutron API
services running (on any role, not just Controller), combined with
whether DVR is enabled (equivalent to current t-h-t logic).
To avoid breaking the NeutronL3HA parameter in t-h-t we maintain an
optional parameter to override the calculated value.
Change-Id: I01c50973eec8138ec61304f2982d5026142f267c
Partial-Bug: #1629187
(cherry picked from commit 6ae718560b)
87e206942d