Supply org and space environment variables to aviary

* Avoids hardcoding these as 'pivotal' and 'coal-mine'
Remove pointless space summary in aviary canary
Separate ping checking into separate aviary class.
Add ping! method to InstancePinger

* pinged_running_ratio no longer has side effects
Rename InstancePinger#pinged_running_ratio to #running_ratio
Add domain and instance count arguments to InstancesAviary
Split url argument of InstancePinger.new into app name and domain
Add InstancesFromCFAviary aviary class
Store latest ratio from CF aviary check
Add instances_from_cf_aviary endpoint for CF-only check
Expose running ratio for pinged aviary
Instance aviary delegates to separate CF and pinged aviaries