engine: Change loading of VdsGroup to load from cache

Several pieces of code load VdsGroup. This patch modifies them so
that they can read VdsGroup from cache (saving about 3ms per call)

Change-Id: Id09c10f06d9fa053540d3b240d53772d0e42c754
Signed-off-by: lzelkha@redhat.com <lzelkha@redhat.com>