make joyent-minimal brand more flexible

fixes joyent/smartos-live#118

joyent-minimal and joyent brands now share the
/zones/manifests/joyent lofs mount

This is safe because the minimal brand overwrites the SMF repository of
whatever image is being used with its own minimal seed repository which
disables early-manifest-import and manifest-import

This change allows a joyent-minimal zone to import any other service
available to the joyent brand without maintaining a complicated brand
manifests file