Improve the documentation of query output fields

The gnt-node and gnt-instance list commands have a customizable list of
output fields, but the list is not up to date (in the man page) and not
easily understandable from the ‘--help’ output.

This patch updates the man pages and adds the available fields and
default fields in the ‘--help’ output, as part of the description.

Example:
Usage
=====
  gnt-node list

Lists the nodes in the cluster. The available fields are (see the man page for
details): name, pinst_cnt, pinst_list, sinst_cnt, sinst_list, pip, sip,
dtotal, dfree, mtotal, mnode, mfree, bootid. The default field list is (in
order): name, dtotal, dfree, mtotal, mnode, mfree, pinst_cnt, sinst_cnt.

Reviewed-by: imsnah,ultrotter