[FEATURE] Read-only filemounts

With this patch a new "read_only" property for filemounts is introduced.

If this property is set to TRUE the user will only have read access
to this filemount. Write operations are prohibited.

Resolves: #49391
Releases: 6.3
Change-Id: I0987114ad56857b4a91f6a2198fd8532c292034b