Fix bug with az containerapp create

[StrawnSC]

Fixes Azure#5253

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[panchagnula]

@StrawnSC I would assume this was added in first place for a reason - would there be any issues with removing this - that might need to be handled separately.

[StrawnSC]

No, this was a last-minute change that we made in the last version because we thought it would make the user experience better. We had the faulty assumption that the user would always want to set a registry server value if they pass in an image

[StrawnSC]

For context, all our testing of the MSI feature for az containerapp create involved using ACR registries like so:
az containerapp create --image sstrawn.azurecr.io/image:tag --registry-server sstrawn.azurecr.io
Based on that usage of the create command, it seemed like we could easily help the user our by trying to parse out the registry server from the --image value. For anonymous registries, the user doesn't need to add the --registry-server value, so this feature broke that use case

[panchagnula]

This looks good - only ask like we discussed is to add a test for anonymous registries so we can catch any issues in our testing.