cloud-init will keep a 'status' file up to date for other applications
wishing to use it to determine cloud-init status.

It will manage 2 files:
 status.json
 result.json

The files will be written to /var/lib/cloud/data/ .
A symlink will be created in /run/cloud-init. The link from /run is to ensure
that if the file exists, it is not stale for this boot.

status.json's format is:
   {
    'v1': {
     'init': {
       errors: []      # list of strings for each error that occurred
       start: float # time.time() that this stage started or None
       end: float # time.time() that this stage finished or None
     },
     'init-local': {
       'errors': [], 'start': <float>, 'end' <float> # (same as 'init' above)
     },
     'modules-config': {
       'errors': [], 'start': <float>, 'end' <float> # (same as 'init' above)
     },
     'modules-final': {
       'errors': [], 'start': <float>, 'end' <float> # (same as 'init' above)
     },
     'datasource': string describing datasource found or None
     'stage': string representing stage that is currently running
              ('init', 'init-local', 'modules-final', 'modules-config', None)
              if None, then no stage is running.  Reader must read the start/end
              of each of the above stages to determine the state.
   }

result.json's format is:
   {
    'v1': {
     'datasource': string describing the datasource found
     'errors': [] # list of errors reported
    }
   }

Thus, to determine if cloud-init is finished:
 fin = "/run/cloud-init/result.json"
 if os.path.exists(fin):
   ret = json.load(open(fin, "r"))
   if len(ret['v1']['errors']):
     print("Finished with errors:" + "\n".join(ret['v1']['errors']))
   else:
     print("Finished no errors")
 else:
   print("Not Finished")