<install> refactor rhsm+rhn conf; preflight checks

rhsm repo conf options outsourced to a function so maintenance requires
only changing the repo name in one place.
Same thing for "rhn" method; also made this idempotent so can rerun the
install without filling up the plugin conf.
Preflight checks now look for user/pass on rhn/rhsm (now consolidated
into common var names).
Also removed a minor preflight problem where rpm -V fails for no good
reason on a re-install; used plain rpm -q instead to test rpmdb.