Add a molecule template to cover the EGI deployed OS's

[brucellino]

Short Description of the issue

The default molecule test scenario only generates a test case for the centos7 OS. We need to add other OS's in order to comply with the EGI infrastructure profile. This test coverage is required by the style guide, but can be deviated from in special cases (see #14). However, it should be opt-out, not opt-in.

Summary of proposed changes

Add a means to a generate the coverage for the role.

[brucellino]

It is apparently possible to generate a molecule scenario from a specific template : ansible/molecule#416

This requires a cookiecutter template repo but I can't figure out yet how it should be done. which is used in template.py. We need to create our own template repo which is used during the generation. More than one template can be used. An example is provided.