Fix simple db image_get to look like sqlalchemy
Fix simple db image_get to look like sqlalchemy

image_get in simple db was repeating logic that exists in helper
is_image_visible method. Also, sqlalchemy implements the same way, so
now this method behaves in both dbs.

Fixes bug 1077256

Change-Id: I5285027a24157750d7c322990aa4074e2b1087c7

openstack/glance,citrix-openstack-build/glance,SUSE-Cloud/glance,akash1808/glance,sigmavirus24/glance,saeki-masaki/glance,vuntz/glance,cloudbau/glance,rickerc/glance_audit,klmitch/glance,redhat-openstack/glance,openstack/glance,tanglei528/glance,JioCloud/glance,ntt-sic/glance,sigmavirus24/glance,tanglei528/glance,saeki-masaki/glance,JioCloud/glance,akash1808/glance,citrix-openstack-build/glance,vuntz/glance,ozamiatin/glance,rajalokan/glance,SUSE-Cloud/glance,rickerc/glance_audit,dims/glance,dims/glance,redhat-openstack/glance,darren-wang/gl,ntt-sic/glance,scripnichenko/glance,jumpstarter-io/glance,paramite/glance,cloudbau/glance,stevelle/glance,ozamiatin/glance,kfwang/Glance-OVA-OVF,openstack/glance,stevelle/glance,wkoathp/glance,darren-wang/gl,kfwang/Glance-OVA-OVF,klmitch/glance,rajalokan/glance,jumpstarter-io/glance,scripnichenko/glance,paramite/glance,wkoathp/glance,takeshineshiro/glance,takeshineshiro/glance