ipagroup: Correctly handle externalmember in member actions

When creating the lists of external members, the attribute
'ipaexternalmember' also needs to be added to the list of external
members that are part of the group object for external groups.

A test to verify the correct behavior was added and the test suite for
group external members have been cleaned up with 'yes' values changed to
'true' and the use of module_defaults.