Improve CLI functional tests

We fix a bug connected to wrong task configuration used in CLI tests for tasks.
We also refactor the tests a bit to make them more readable.

CLI functional tests have been moved to a separate directory, and the tests_ci/
directory has been supplied with a README file.

Co-authored-by: Sergey Skripnick<sskripnick@mirantis.com>
Change-Id: I927c91f119765f27ee6b8b7c98d4156c02700102
Closes-Bug: 1374407
Merge "Improve CLI functional tests"