webadmin: optimize system tree queries.

- Previously the system tree would sequencially call
  methods to retrieve the data once the previous call
  was finished. This causes at least 7 distinct queries
  one after another. This patch optimizes the queries to
  3 distinct queries and some in parallel.

Change-Id: I304585d93de970030095919614ec13b2f46fc0ef
Signed-off-by: Alexander Wels <awels@redhat.com>