automember: Verify condition keys

The automember_add_condition and automember_remove_condition commands
are not verifying condition keys in all cases. This is for example not
done in the removal case if a condition is not part of the automember
rule.

For consistent behaviour this needs to be done in the automember module
now. The condition keys are verified with the user and group aciattrs
returned by the API command json_metadata now.

Related: RHBZ#1976926