Support for zypper parallel preload (ZYPP_PCK_PRELOAD and ZYPP_CURL2 environment variables).

[arvidjaar]

See https://lists.opensuse.org/archives/list/factory@lists.opensuse.org/thread/LOCZIG43MFJSTUIQ3VH2CRSYRCBNR4O7/#JZXNN2N77IOOZVQZDVGVC3AK5U2XEY4U

There are two issues

1. tumbleweed script is using sudo so environment variables get lost. It needs to pass ZYPP_PCK_PRELOAD and ZYPP_CURL2 through.
2. Using metalink= may improve download speed, tumbleweed needs to support editing this URL in addition to baseurl=.

[boombatower]

1. hmm, if you do tumbleweed switch and then run zypper however you like that should work to do zypper ref and zypper dup. Not sure what the best solution is as a fork to not use sudo or passing in variables seems not ideal. Could just use sudo -E, but not sure if any other negative side effects of doing that.
2. tumbleweed-cli would need to modify the URL on initial setup, but after that the libzypp variable is used to change the snapshot. If you modify the file yourself it should work.