Add support for having the data dir on vbox shares

This requires mounting said share with the uid and gid of mysqld.
So we can only do this here as the uid/gid is not set before
the package is installed.
Allow different data dir in apparmor profile

A different data dir has to be allowed in the mysqld apparmor
profile or else it will fail to start as it has no permission
to write and read from there.
Only mount if not already mounted

Test whether the shared data folder is already mounted with the
right permissions and don't mount if it is.