[Feature] add possibility to use another name for cnf config files

[kosstennbl]

Is your feature request related to a problem? Please describe.
cnf-config option can take only files with "cnf-testsuite.yml" name, usage of other filenames causes failures or leads to very unexpected behavior.

Describe the solution you'd like
Possibility to use different names for cnf config file. Some solution would be needed for constants and "CNFManager.cnf_config_list" function. Easiest one (but not a nice one) would be allowing user to use their config name, but rename it to "cnf-testsuite.yml" on copy to internal cnfs/* directory.

Describe alternatives you've considered
Leave limitations of config naming as it is now, but give user clear error messages and don't allow him to use any other name for cnf-config parameter. Error should be given at the earliest stages so no actions would be made by the application.

**How will this be tested? aka Acceptance Criteria (optional) **

(optional: unnecessary for things like some version bumps etc.)

Once this issue is addressed how will the fix be verified?

[kosstennbl]

Duplicate of #1429
Should be further discussed there.