Additional deploy validation

[ssalinas]

Two main pieces:

• Add the ability to lock down artifact specification to only signed artifacts (s3 + signature file). This does not validate everything within a list, assuming that if the list is signed, the things within the list are valid
• Add valid docker registry list so Singularity can lock usage down to only an internal docker registry if needed

[pschoenfelder]

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[pschoenfelder]

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