hbacrule: Fix member management idempotence issues.

Members of hbacrule must be compared in a case insensitive manner.
This patch fixes comparation of member parameters against existing
members by converting parameters to lowercase.

Also, there were some cases where a change with an empty set of members
was issued to IPA API, leading to a result of 'changed: yes' when
'changed: no' was expected. The fix involved a refactoring of the
member management code.