Add support for group trigger deletion in hawkular alerts

[dkorn]

Adding delete_group_trigger method in hawkular alerts and test_delete_group_trigger to alerts tests.

Since the DELETE action returns only a response code, without body, I added parse_json flag to _http method in hawkular/client.py. Without this addition an error is raised when trying to load
the json with no content.
I'm setting it to True by default, while sending it with False from the _delete method.
This Issue was solved in hawkular/metrics.py by checking that the method is GET, a wrong solution IMHO cause POST and PUT methods also return body.

[dkorn]

@cben @simon3z @rubenvp8510 @enoodle please review

[burmanm]

@dkorn The metrics solution of only GET was usable when the client was created (there were no endpoints that would send response in PUT/POST other than error cases).

Could you rebase this and the other PRs? I wanted to merge the Travis PR before other PRs (for obvious reasons)

[dkorn]

@dkorn The metrics solution of only GET was usable when the client was created (there were no endpoints that would send response in PUT/POST other than error cases).

I understand, makes sense.

Could you rebase this and the other PRs? I wanted to merge the Travis PR before other PRs (for obvious reasons)

Done.

@burmanm we still need to decide on a solution in cases where there is no response body.
possible solutions:

1. parse_json flag (as I used here)
2. read the string with the reader and only if not empty load as JSON
3. catch an exception (ValueError for example)
4. other solution

this was also discussed in #28 (review)

[burmanm]

@dkorn I actually like the parse_json flag, not because it is automatic but because it isn't. When we create methods that expect a JSON to be returned, we've defined some sort of agreement. If the JSON wasn't there or it couldn't be parsed, we really should get an error (instead of silent failure).

[dkorn]

@burmanm I agree, will also change #28 to include the parse_json flag.
Take into account that put method returns body in some cases, so this flag will be passed through other prior methods.

[rubenvp8510]

I reviewed this PR and LGTM, I agree with the change to include the parse_json flag.