Migrate configuration to <plugin-name>.config from gerrit.config

Move existing configuration to <plugin-name>.config as it provides
more flexibility when naming i.e subsection names can be used and
makes them readable. Using a <plugin-name>.config also has the
added advantage of having all plugin related configs in a single
location.

For backward compatibility, existing configuration is still read from
gerrit config if the plugin config is not found.

Change-Id: If0ed517406c3f5a78c8b0328ef5d4c23c8326dae
4 files changed