Fix update.sh invocation with unknown variant
Fix update.sh invocation with unknown variant

If update.sh was invoked with non-existing variant, it updated all
variants.  Change in_variants_to_update to return true always if
second variant argument was not given, but not when given variant
argument does not match to available variants.

joyent/docker-node,chorrell/docker-node,pesho/docker-node,nodejs/docker-node,nodejs/docker-node,chorrell/docker-node