rbd: get path: allow fake override of fsid in scfg for some regression tests
to avoid calls into RADOS connect, that trigger RPCEnv not
initialized breakage in regression tests, but wouldn't really work
otherwise either
in the future the RBD $scfg could actually support this (or similarly
named) property, to safe on storage addition and then avoid frequent
mon commands
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>