Fix ami/aki image create use of $img_property
Fix ami/aki image create use of $img_property

This abstracts out the conversion of key=value,... property lists to a function
and makes both _upload_image() and the two ami/aki image create calls use it. The
move to bare key=value properties introduced a regression for ami/aki where
the --property flag stopped being passed to osc in that case.

Change-Id: Idf7fdfe3f5800f79f6c48f9d9606a7b53436a730

openstack-dev/devstack,nawawi/openstack,openstack-dev/devstack,nawawi/openstack