Support http as store for manifest tarballs

[ripienaar]

Should be able to fetch tarball manifests from HTTP

• like obj://FOO/bar.tgz in ccm apply (#93) Support fetching manifests from HTTP servers in apply #97
• like obj://FOO/bar.tgz in ccm agent, in this case though we should start without monitoring the HTTP server, though might be able to do HEAD requests regularly