Allow configuration of different (i.e. json) escaping for access log

[philippthun]

Thanks for contributing to the capi_release. To speed up the process of reviewing your pull request please provide us with:

• A short explanation of the proposed change:
  With this PR the characters escaping used for logging variables to the nginx access log can be set to json. This allows changing the log format to a JSON string (e.g. {"key":"$variable"}) and getting properly escaped JSON logs.

• An explanation of the use cases your change solves
  With Logstash the parsing of JSON-formatted logs becomes easier and is more flexible (i.e. adding a new field does not require the parser to be changed).

• Links to any other associated PRs

• I have viewed signed and have submitted the Contributor License Agreement

• I have made this pull request to the develop branch

• I have run CF Acceptance Tests on bosh lite

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/177492597

The labels on this github issue will be updated when the story is started.

[sethboyles]

LGTM. Thanks for the PR!