Allow scheduling of update jobs for any arch (#57)
Rather than hardcoding x86_64. I thought of ways to have the
method schedule jobs for multiple arches in one call, but in
the end didn't really like any of them, this should be much
simpler. I don't think there'll be a significant performance
difference either as ultimately there has to be one POST request
per arch/flavor combination - we can't send a POST that creates
jobs for more than one arch at a time.
Fixes: #57