webadmin: Refactor shared storage options on DataCenterGuideModel

Refactored the update of available actions on shared storage in
DataCenterGuideModel, extracted methods, removed redundant code and such

Change-Id: I0ac7b5bd949f659e5a7c008db87c708187744575
Signed-off-by: Tal Nisan <tnisan@redhat.com>