Enabled regex testing for content_match

[chrissnell]

Here's a quick change to Ryan's http_check.py revisions that enable regex testing for content_match. This makes content_match more flexible in accommodating situations where a simple if ... in ... match would be insufficient.

Consider this real-life example of content being tested:

{"status":"UP","rabbit":{"status":"UP"},"db":{"status":"UP","database":"MySQL","hello":1}}

If we want to test for that first "status":"UP", we need to use a regex because that string is not unique within the content string:

   content_match: '^\{"status":"UP"'

The one gotcha with regex in content_match is special metacharacters like "?". I considered creating a content_match_re for regex matching but decided that it would be cleaner to just document in the example YAML the need to escape these characters.

[remh]

Thanks a bunch a @chrissnell !

Our master branch is currently frozen for the preparation of our 5.2.0 release.

We'll review your pull request for our 5.3.0 release.
Thanks again!

[yannmh]

Thanks @chrissnell . I just rebased your PR on the last 5.2.1 agent release and opened a new PR #1390.