Add config to set min/max dhcp agents per network

[Alberto Donato]

It'd be nice to have a couple of charm configs to be set min/max DHCP agents per network, similarly to what happens with the L3 agent with min/max-l3-agents-per-router.

[Alex Kavanagh (ajkavanagh)]

So, I've dug into this, and the min/max request for dhcp_agents_per_network for neutron doesn't seem to fit according to the documentation for neutron.conf (which is written from this charm):

http://docs.openstack.org/liberty/config-reference/content/section_networking-options-reference.html

This indicates that there is a single setting "dhcp_agents_per_network" (with a default of 1).

Note that this IS different to the min/max-l3-agents-per-router, which is configured like that in the neutron.conf. See:
http://docs.openstack.org/liberty/config-reference/content/networking-options-l3_agent.html

Therefore, I propose that a single option "dhcp-agents-per-network" is added (with a default of 1).

[Alex Kavanagh (ajkavanagh)]

After a conversation with the bug author on IRC, we've agreed to implement the dhcp-agents-per-network option with a default of 2, but ONLY activate it when the the 'enable-l3ha' is set.

[Nobuto Murata (nobuto)]

For example VLAN provider network with enable-local-dhcp-and-metadata enabled in neutron-openvswitch charm, dhcp-agents-per-network is still helpful even without L3 functionality.

[Alex Kavanagh (ajkavanagh)]

Okay, this is a bit back and forth: However, having discussed nobotu's requirement, it does seem that there is a need for dhcp_agents_per_network to be settable orthogonally to l3ha.

The openstack docs indicate that it has a default setting of 1, so adding it such that it always defaults to 1 (if not explicitly set) will have no impact on existing bundles/settings.

The only downside is having to remember to set dhcp_agents_per_network if you want it in ADDITION to l3ha. However, I believe it is the best of both worlds.

Thus, the setting will be available regardless of 'enable-l3ha'.