Default neutron dhcp_agents_per_network to number of agents

Default neutron dhcp_agents_per_network to number of agents

This patch will set neutron's dhcp_agents_per_network equal to the
number of deployed neutron DHCP agents unless otherwise explicitly set.

Conflicts:
	manifests/profile/base/neutron.pp

Note: spec/classes/tripleo_profile_base_neutron_spec.rb removed from
backport as it required defining the neutron class as a precondition to
satisfy a requirement for a rabbit password. This leads to a duplicate
definition.

Partial-bug: #1632721
Change-Id: I5533e42c5ba9f72cc70d80489a07e30ee2341198
(cherry picked from commit 52a68ffc8f060e1961458a524e5861cea02d1c1c)