[sensu-api-checks] add working nova,glance api sensu checks

These changes add two files to the sensu client's plugin path of a controller node:
* check-os-api.rb: a generic plugin to query various openstack API endpoints
* keystone_auth_token.rb: a library required by check-os-api.rb to obtain an auth token from keystone 

This commit also configures the following api health-checks for a controller node:
* check-nova-api: queries nova-api, located at the cluster's fqdn, for a list of servers
* check-glance-api: queries glance-api, located at the cluster's fqdn, for a list of images