Prevent creation of http images with invalid URIs

Active images should always be ready to be downloaded, regardless
they're locally or remotely stored. This patch prevents images with
invalid location URIs from being created and entering the 'active'
state with no data. This is only for the HTTP store.

Closes-Bug: #1257273
Co-authored: Jon Bernard <jobernar@redhat.com>
Change-Id: Iffce79b654cabe8397c85b2cc50c4b7f59733ea5