Need a way to distinguish staging images from production images for timestamp-based tags

[mthalman]

The tagging pattern used for the images in this repo is the following:

dotnet-buildtools-prereqs-docker/src/ubuntu/manifest.json

Lines 13 to 14 in b85260c

	"ubuntu-18.04-$(System:TimeStamp)-$(System:DockerfileGitCommitSha)": {},
	"ubuntu-18.04$(FloatingTagSuffix)": {},

The FloatingTagSuffix variable is only applied to the floating tag. The FloatingTagSuffix variable is what indicates whether the tag is staging or production (it's set to staging if a staging image or (blank) if production). But for the timestamp-based tags, there's no such distinction. So when people do want to use a timestamp based tag, they'll look them up in https://mcr.microsoft.com/v2/dotnet-buildtools/prereqs/tags/list and see a bunch of them listed but there's no way to tell if it's from staging or production.

I think the appropriate thing to do is rename FloatingTagSuffix to something like QualityTagSuffix and apply it to both tag types.

[mthalman]

[Triage]
It would be best if the variable wasn't actually a suffix but rather inserted in the tag value to help better organize the list of tags.

e.g.
"ubuntu-18.04$(Quality)-$(System:TimeStamp)-$(System:DockerfileGitCommitSha)": {},

Defining it this way would group all the "staging" tags together for a given image type, for example, when listed alphabetically.