snf-ci: Bulid pithos-web-client package
Synnefo depends on pithos-web-client. Up until now we were downloading this package from apt.dev.grnet.gr. But when we make a change in Synnefo that also affects pithos-web-client, we have to upload a new version of pithos-web-client. This patch instructs snf-ci to clone and build the latest pithos-web-client.