Support zero-length cluster_name
Support zero-length cluster_name

Currently, if an empty cluster name is supplied to compose, it will
attempt to generate invalid box and host names starting with '-'.

This patch changes the behavior to only prefix boxname and hostname with
"#{cluster_name}-" if cluster_name is non-nil and not the empty string.

fabriziopandini/vagrant-compose,fabriziopandini/vagrant-compose