Use glance image-show to check for uploaded Docker images

The behavior of the code being replaced was failing
with '-o errexit' should that, as in the common case,
the image has not been uploaded into Glance.

While we could workaround this using a '|| :', the existing
code also happened to overwrite the DOCKER_IMAGE global
which is used elsewhere. It seemed prudent to either change
this variable name or remove it altogether.

Finally, using 'glance image-show' is more deterministic
than grepping the output of 'glance image-list'.

Change-Id: I23188155966ae9db64259b4a9d25a0d98c63c912
Closes-Bug: 1286443
Merge "Use glance image-show to check for uploaded Docker images"