Allow IPA_ENABLED_MODULES to override automatically selected ones.

[rjeffman]

When modifying ansible_module_utils, all playbook tests are executed, although some changes only affect a few modules.

One way to speed up the CI tests is to ensure only the affected modules are tested, for example, with a TEMP commit selecting only the modules that are known to be affected.

The only way to achieve this now is to list all but the selected modules in IPA_DISABLED_MODULES, which is tedious, error prone, and tend to grow as more modules are added.

What is requested is that either IPA_ENABLED_MODULES override the automatic selection, or that IPA_DISABLED_MODULES allow for a complement of the existing set (all minus the selected ones).