Add checks for not set sto and sgo bundle paths

[vkmc]

This change adds a check for whether sto and sgo bundle paths are defined and properly inform the user if they are undefined.

Also sets the nightly builds available in Quay as defaults.

[elfiesmelfie]

This looks good to me, I'll rebase the efoley-add_zuul branch on top of this and see if it all passes CI.
Once that happens, we can merge this.

[elfiesmelfie]

I was seeing some issues with the gate, which have since been resolved.
I've rebased PR- 429 onto this branch, and we'll see the results soon.
While the job is running, you can follow the console output here:
https://review.rdoproject.org/zuul/stream/63f12f83c3324b60a54b0a33fe957795?logfile=console.log

I removed the bundle image paths from ci/deploy.yml. I expect that the job still passes, since this change takes care of that case where those values are unset.

Zuul passed for the branch that includes these changes.
https://review.rdoproject.org/zuul/buildset/3d1acad5cfdf40248eb7b387039732b9

[elfiesmelfie]

Zuul passed: https://review.rdoproject.org/zuul/buildset/3d1acad5cfdf40248eb7b387039732b9

The commits in this review are included in that passing branch.