Add AllowIsolate=yes to snapshots
core/socket: log errors when starting socket for this socket

When showing an error like 'Socket service not loaded', the
error won't show up in the status for the socket, unless it is
marked as SYSTEMD_UNIT=*.socket. Marking it as SYSTEMD_UNIT=*.service,
when the service is non-existent, is not useful.
cryptsetup: RequiresMountsFor if source is a file

Fixes: https://bugzilla.novell.com/show_bug.cgi?id=730496
       https://bugs.freedesktop.org/show_bug.cgi?id=60821
cryptsetup: add RequiresMountsFor for keyfile

This ensures that the keyfile is available during the opening of the encrypted
device.

Also dropped the explicit ordering Before=local-fs.target, as the containers
are ordered implicitly by their content.