Introduce proxy service, which allow only http(s) traffic to yum repos. The
filter rules are based on URL regexp, so it isn't full-featured content
inspection and can be easy bypassed, but should be enough to prevent some
erroneus user actions (like clicking on invalid link).
It is set up to intercept connections to 10.137.255.254:8082, so VM can connect
to this IP regardless of VM in which proxy is running. By default it is
started in every NetVM, but this can be changed using qvm-service or
qubes-manager (as always).
