[SPARK-4048] Enhance and extend hadoop-provided profile.

This change does a few things to make the hadoop-provided more useful:

- Create new profiles for other libraries / services that might be provided by the infrastructure
- Simplify and fix the poms so that the profiles are only activated while building assemblies.
- Fix tests so that they're able to run when the profiles are activated
- Add a new env variable to be used by distributions that use these profiles to provide the runtime
  classpath for Spark jobs and daemons.
I did not see any direct dependency on jetty from the streaming code. I'll double check, though.