Fix deadlock starting 3tier app

Builds http://brooklyn.builds.cloudsoftcorp.com/job/Brooklyn-Master-Integration/373 and http://brooklyn.builds.cloudsoftcorp.com/job/Brooklyn-Master-Integration/375 were both hit by this.

Taking description from first commit message:

- Calling getShellEnvironment() during jboss.install() was causing it
  to lookup the config, including sysprops. However, that required
  MySql to have a URL before it could resolve.
  On localhost (e.g. integration tests), the jboss.install() held a
  lock (SshMachineLocation.acquireMutex) which meant only one thing
  could be installed at a time - hence prevented MySql from being 
  installed. Therefore deadlock.
- Fixed by guarding the call to getShellEnvironment(), to only be
  done if env was not explicitly supplied.