webadmin: Cleanup of AdElementListModel constructor

Cleanup part of the bugfix patch

Nested async call moved to be parallel as it was not dependent
of its parent

Async calls logic encapsulated from mid method anonymous class
to its own method

Improved variable names for readability

Change-Id: Iacc5878f32f5b5395dd5a87a0614b4c3a21bf88d
Signed-off-by: Noam Slomianko <nslomian@redhat.com>