Add an option to run puma in upstart

At the moment monit seems to be the only way to start puma provided with cookbook. This pull request offers a simple option to run puma with upstart. It's disabled by default so will not affect existing setups.

Note #1: this option is mutually exclusive with monit to avoid possible conflicts.
Note #2: may be especially useful with JRuby where there's no need to run multiple puma instances due to lack of GIL.