Review dependencies for stream configuration

[sbesson]

See #33 and the original rationale for this PR

Recent changes in the nginx distribution removed support for installing all nginx modules. Versoin 0.15.2 of this role disables the stream block in the top-level configuration file if no stream configuration is defined. With the removal of the public rsync in IDR, this is now the case across all OME playbooks.

My understanding is that the stream configuration using this role is currently broken by default on RHEL 7 systems at least. A first step towards a fix might be to review and isntall some module dependencies conditionally to the presence of stream role variables.