neutron_dhcp_agent container doesn't define a default value for package list

[Brent Eagles]

The fix for https://bugs.launchpad.net/kolla/+bug/1753568 allowed the package list to be set on CentOS by defining neutron_dhcp_agent_packages. For consistency it should've set a default value of an empty list and allowed overriding via neutron_dhcp_agent_packages_append.

[Michal Nasiadka (mnasiadka)]

You can't use append, but you can override this variable - even to an empty value and _append to it later in template-override.