Avoid uploading trove guest image twice

When creating the trove datastore, get the guest image id from glance.
The trove guest image gets uploaded along with all the other
images specified in $IMAGE_URLS. It is not necessary to upload
it again in init_trove.

Also: add xenapi case to stackrc for trove guest image

Closes-Bug: 1339818
Change-Id: I2973af27ab93182fcd674f874daba1036d767d52
Merge "Avoid uploading trove guest image twice"