subtests.docker_cli: Update documentation docstrings

Hi @cevich, for each test I checked the prerequisites and removed the `A remote registry server` as this one is true for every test...

Note that I did not remove the configuration sections when they were filled as it will be easier to move them to INI file when the grabber is accepted. Although I have not create it when it was missing as it will be easier to do that in INI file later.

Couple of times I updated the description, but usually it was sufficient. I have not add any SubSubtest-related descriptions as those test's SubSubtests differed only in variables, not in steps. With the INI parser we'll have the `subsubtests = ...` list as first description in the `configuration` section so it will be easy to see which SubSubtests are there.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>