The following variables are added:

* prometheus_blackbox_exporter_cmdline_extras
* prometheus_elasticsearch_exporter_cmdline_extras
* prometheus_haproxy_exporter_cmdline_extras
* prometheus_memcached_exporter_cmdline_extras
* prometheus_mysqld_exporter_cmdline_extras
* prometheus_node_exporter_cmdline_extras
* prometheus_openstack_exporter_cmdline_extras

Change-Id: I5da2031b9367115384045775c515628e2acb1aa4