CLUSTER-4. Allow users to run port configuration utility at any time.

This commit modifies bento-cluster so that its port configuration utility
can be run by a user via the bento command `bento config`.  There are also
some small changes for cleaner code / better looking output.

The port configuration utility will retain any non-port configuration in
the Hadoop/HBase resource XML files if the --keep-current-conf flag is
true (default).  The utility will now also always prompt the user for
ports, even if the defaults are open, if the flag --always-prompt=true is
used (false by default).

Additionally, if Hadoop/HBase resource XML files exist, any port configuration
existing in those files will be used as the default values for ports by the
utility.