gcp provider report subcommand

[kapilt]

report on gitter, not sure this has been explicitly supported yet, the meta model for gcp provider is a bit different.

custodian report -s out gcptest.yml
Traceback (most recent call last):
File "/home/custodian/bin/custodian", line 10, in <module>
sys.exit(main())
File "/home/custodian/lib/python2.7/site-packages/c7n/cli.py", line 368, in main
command(config)
File "/home/custodian/lib/python2.7/site-packages/c7n/commands.py", line 136, in _load_policies
return f(options, list(policies))
File "/home/custodian/lib/python2.7/site-packages/c7n/commands.py", line 276, in report
policies, begin_date, options, sys.stdout, raw_output_fh=options.raw)
File "/home/custodian/lib/python2.7/site-packages/c7n/reports/csvout.py", line 75, in report
include_policy=len(policy_names) > 1
File "/home/custodian/lib/python2.7/site-packages/c7n/reports/csvout.py", line 152, in init
self._id_field = model.id
AttributeError: type object 'resource_type' has no attribute 'id'
019-02-28 20:13:55,280: custodian.policy:INFO policy: serverless-ftw resource:gcp.instance region: count:38 time:0.42
it was a open pull for gcp.instance