rbd: make monhost option optional

rbd: make monhost option optional

to allow differentiating between user-created external RBD storage
entries (WITH monhost), and those created and managed by pveceph
(without).

making monhost non-fixed allows easily opting into the managed behaviour via
'pvesm set STORAGE -delete monhost', but is also helpful for external clusters
(i.e., after adding or removing a monitor you need to update the monhost
parameter..)

adapt description accordingly.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>