Support in MountService to get a (preview) mount by id

Currently, if you want a preview mount by id you need something like

MountService#getPreviewMounts(hostGroup).get(mountId);

The above can be inefficient since always all mounts are first fetched. Next to that, you always need the host group. However, if you have the mount id, you can get the mount without host group. Hence I like to add

Mount getLiveMount(final String mountId);
Mount getPreviewMount(final String mountId);