[beeswax] Improve secured cluster configuration experience

https://github.com/cloudera/hue/issues/12

hive query uses default principal hue/localhost@<REALM> in secure cluster

I configured CDH4 with kerberos and my hive queries failed until I added beeswax_server_host value and tweaked beeswax/db_utils.py to support my non-standard hue principal name (needed to comply with my company's kerberos naming conventions).

I didn't discover the beeswax_server_host configuration option until digging through code. Since the default of localhost isn't great for a secured cluster I'd request that the beeswax_server_host configuration option be added in the Beeswax section with some comment that it might be needed for secure clusters. Further, the call thrift_util.get_client in beeswax/src/beeswax/db_utils.py uses a hardcoded kerberos_principal value which doesn't accommodate kerberos configurations which need to make use of kerberos principal to short name mapping such as is possible through "hadoop.security.auth_to_local".