provide alternate image path for install

[eschrock]

Problem

If a user doesn't have access to docker hub due to firewall issues, etc, they have no way of installing titan because the image path is hard-coded.

Solution

Provide titan install [-i image] to specify an alternate image path. This would let them go through their own offline certification process to vet the titan images, host in a private registry, etc.

Alternatives

I believe they can address this by manually pulling an image and tagging it as titan:latest, but it might be nice to have it as a first-class option.

[mcred]

Should the CLI still be in charge of the correct version? -i myregistry:titan vs -i myregistry:titan:0.4.0