hbacsvcgroup: Fix member management idempotence issues.

The hbacsvc members of hbacsvcgroup must be compared in a case
insensitive manner. This patch fixes comparation of member parameters
against existing members by converting parameters to lowercase, as it
is how the hbacsvc members are stored for hbacsvcgroups.

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
hbacsvcgroup member management code.