rbd: get path: allow fake override of fsid in scfg for some regression tests

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>