Allow configuration of Nova Server admin properties

All OS-EXT-SRV-ATTR:.* properties are filtered out from search results
for non-admin users. This is not a configuration option in the Liberty release.
To change this or filter out additional properties, you must change the
plugin code to add additional properties.

See: ADMIN_ONLY_PROPERTIES in searchlight/elasticsearch/plugins/nova/servers.py

This should be updated to be configurable via the plugin config in searchlight-api.conf.

E.g.

    [resource_plugin:os_nova_server]
    admin_only_properties = ...