rbd: Utilize storage pool namespace to manage config options

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.html

Signed-off-by: John Ferlan <jferlan@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>