Configure plugins by name

To configure core plugin or service plugins, instead of using class names,
simpler names can also be used, ex:

service_plugins = router, firewall, lbaas

For compatibility with previous versions, the class name of a plugin can be
specified instead of its entrypoint name, ex:

service_plugins = router, neutron.services.firewall.fwaas_plugin.FirewallPlugin, lbaas

DocImpact

Implements: blueprint config-plugin-by-name
Change-Id: Ia4aaa1d305b160a4d6dab9e227c744727a4c78c2
Merge "Configure plugins by name"