[#71591898] Move schemas to constants under Launcher::Schema

This follows the pattern in use by alphagov/vcloud-edge_gateway, which I
like because it makes it clear that these don't get computed or change at
runtime and allows us to point users at them if they need to know what a
valid configuration looks like.

Like alphagov/vcloud-edge_gateway@b30f91b I have also included a link in the
README so that people can find them.

PS: Additional newline in VmOrchestrator to match other classes.