pbs client: default to configured namespace for non-namespaced parameters

pbs client: default to configured namespace for non-namespaced parameters

For get_snapshots(), also set the default when no namespaced parameter
is present at all.

This would break any callers that have a namespace in the initial
config and explicitly don't set it for a later call, but the only
such caller is restore_pxar() in PMG, which /should/ be using the
namespace!

In other words, this implicitly fixes the restore_pxar() call in PMG
and avoids the need to extract the namespace from the configuration
(which already is present in the client) on the call site for all
functions that currently take a namespaced parameter.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>