Extensions and fixes to ci's functionality

This patchset aims to extend the testing functionality that ci provides as well as offer a few fixes.

Briefly, the first patch allows to run more fine-grained tests. The user can now run tests specifically for an app by giving the `<component name>.<app name>` as an argument, e.g. `./ci/tests.sh astakos.im`.

The second patch allows `snf-ci` to work with Images with many users. It does so by always choosing the first user on the list.

The third patch is pretty simple as it updates the default grnet repo that `snf-ci` uses.

Note: this patchset has been checked and passes the tests.