Allow for adjustment of RBD configuration options via Storage
Pool XML Namespace adjustments. When namespace arguments are
used to start the pool, add a VIR_WARN to indicate that the
startup was tainted by custom config_opts.

Based off original patch/concept:

https://www.redhat.com/archives/libvir-list/2014-May/msg00940.htmlSigned-off-by: NJohn Ferlan <jferlan@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>