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>

PVE/Storage/RBDPlugin.pm

@@ -258,7 +258,7 @@ sub plugindata {
 sub properties {
     return {
 	monhost => {
-	    description => "Monitors daemon ips.",
+	    description => "IP addresses of monitors (for external clusters).",
 	    type => 'string', format => 'pve-storage-portal-dns-list',
 	},
 	pool => {
@@ -284,7 +284,7 @@ sub options {
     return {
 	nodes => { optional => 1 },
 	disable => { optional => 1 },
-	monhost => { fixed => 1 },
+	monhost => { optional => 1},
 	pool => { optional => 1 },
 	username => { optional => 1 },
 	content => { optional => 1 },