Add HasValidGITPrefix to utils/utils.go

This will allow us to use a common Git prefix check for both api/clients/commands.go and
builder/job.go. Previous prefix check in build from Git (in builder/jobs.go) ignored valid prefixes such as "git@", "http://" or "https://".

Signed-off-by: Lakshan Perera <lakshan@laktek.com>