Make neutron dhcp agents per network conditional

Make neutron dhcp agents per network conditional

While the heat templates specify a default value of 3, it rarely seems
to have an effect as the tripleoclient is setting this according to the
controller scale. This was fine before composable roles, but it is now
invalid. While the client needs to be modified to no longer set this
according to controller scale, the template should default to a sentinel
value that will allow the puppet code to determine the proper value by
the number of hosts that have the neutron dhcp agent deployed on them.

Depends-On: I5533e42c5ba9f72cc70d80489a07e30ee2341198
Partial-bug: #1632721
Change-Id: I06628764c4769d91bbc42efe1c722702d6574d02
(cherry picked from commit 3c5345fc75da1e289929ef5caf08a0f75f904bb4)