use cookie based authentication as default authentication method

[scrat14]

Since check_rhev3 1.2 it's possible to use cookied based authentication instead of username and password based authentication for RHEV >= 3.1 and oVirt >= 3.1 environments when specifiying option "-o".

As most users didn't recognize this options. As discussed in oVirt mailling list, it's better to use cookied based auth first and fallback to username and password.

Workflow for this could be:

[scrat14]

Commit 394dc00 needs to be tested...