spec: allow specifying uid/gid/mode of mount points

[cgonyeo]

@omkensey wants to be able to specify the uid:gid owner of a mount point and its mode. Setting a pre-start event handler to chown the directory after an ACE has mounted the volume is insufficient, as the user that command runs under may not have the necessary permissions.

Here's the acbuild issue asking for this: containers/build#194