replace type calls with isinstance
replace type calls with isinstance

In store/__init__.py, there are two uses of the "type" method to
check type compatibility. It is better to use the "isinstance" method
since it will work for subtypes as well.

Change-Id: I20958aa449822cac2cb03e12557fded3f45a9e05
Closes-Bug: #1267074

stevelle/glance,takeshineshiro/glance,vuntz/glance,rajalokan/glance,JioCloud/glance,klmitch/glance,sigmavirus24/glance,openstack/glance,dims/glance,kfwang/Glance-OVA-OVF,saeki-masaki/glance,openstack/glance,JioCloud/glance,scripnichenko/glance,stevelle/glance,wkoathp/glance,paramite/glance,darren-wang/gl,openstack/glance,akash1808/glance,jumpstarter-io/glance,tanglei528/glance,akash1808/glance,ozamiatin/glance,redhat-openstack/glance,redhat-openstack/glance,saeki-masaki/glance,kfwang/Glance-OVA-OVF,tanglei528/glance,vuntz/glance,dims/glance,klmitch/glance,scripnichenko/glance,paramite/glance,jumpstarter-io/glance,sigmavirus24/glance,takeshineshiro/glance,wkoathp/glance,rajalokan/glance,darren-wang/gl,ozamiatin/glance