Make nginx timeout configurable

[johha]

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

• A short explanation of the proposed change:
  Currently the nginx timeout for graceful shutdown is hard coded to 30 secs. If requests take longer than this (e.g. v2/services on big foundries) they'll be terminated. Making this configurable will allow operators to prevent (too many) failed requests during monit restarts.

• Links to any other associated PRs
  PR for capi-release: Shutdown nginx timeout capi-release#188

• I have reviewed the contributing guide

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

• I have made this pull request to the main branch

• I have run all the unit tests using bundle exec rake

• I have run CF Acceptance Tests

[cf-gitbot]

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

https://www.pivotaltracker.com/story/show/177160167

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

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Johannes Haaß (295daea)