ipahostgroup: Fix idempotence issues due to capitalization

ipahostgroup parameters 'host', 'hostgroup', 'membermanager_user' and
'membermanager_group' must be compared in a case insensitive manner
and stored as lower case strings.

This patch fixes the comparison and storage of this parameters, and
change the handling of members to use the same structure as in newer
modules.

Two new tests files were added:

    tests/hostgroup/test_hostgroup_case_insensitive.yml
    tests/hostgroup/test_hostgroup_membermanager_case_insensitive.yml