[Bug 34618] do not show low disk space notification for read-only volumes

[Martin Pitt]

Public bug report changed:
https://launchpad.net/malone/bugs/34618

Comment:
the configurability is already in bug #33967, that's why I initially
marked this as a dup. However, the "do not show low disk space
notification for read-only volumes" is something new and makes sense,
thus I devote this bug to this part.