Make Nginx healthcheck script to tolerate unavailability

[a-b]

• A short explanation of the proposed change:
  Addresses Healthcheck script exits when nginx is not available resulting deadlock #364 health check will wait and retry if Nginx is not available.

• An explanation of the use cases your change solves
  This change allows Nginx and webserver to start without monit interruption.

• 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

[Gerg]

What is special about curl exit code 7? Would this same deadlock behavior not occur for other curl exit codes? It would be helpful to have details about this error case in the commit message.

Also, if we only care about the "Connection Refused" case, would it be more appropriate to use the --retry-connrefused curl flag?

[sethboyles]

I think there was hesitation to use --retry-connrefused as it was added to curl 7.52.0, and ubuntu-xenial stemcells (apparently) contain curl 7.47.0.

[a-b]

What is special about curl exit code 7? Would this same deadlock behavior not occur for other curl exit codes? It would be helpful to have details about this error case in the commit message.

Updated commit message.
Do you have any particular scenarios for other curl exit codes that you're worried about, @Gerg?