Collapse --external-ca-profile tests into single class
To avoid having to spawn new CI hosts for each kind of
--external-ca-profile argument we are testing, collapse the three
separate test classes into one. Uninstall the half-installed IPA
after each section of tests.
This change is in response to review comment
https://github.com/freeipa/freeipa/pull/2852#pullrequestreview-220442170.
Part of: https://pagure.io/freeipa/issue/7548
Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>