group ID in loci install script should be configurable

In some instances, an operator may want to compose OpenStack project installations. For example, in installing the Ironic bare metal driver a nova-compute node will need an Ironic installation so it can import the ironicclient module. It currently not possible to reuse the Loci install scripts to install multiple projects because the group and user names are hard coded to 42424. This should be an environment variable that one can override at build time to allow for new layers to install additional projects from source.