e2e: allow templating in test suite variable files

[askervin]

Test suite directories contain files varname.var* from which
environment variables are created for run.sh. This extension enables
using framework's common templating in variables when the file is
named "varname.var.in.*". Templating allows, for instance, expanding
environment values and giving default values when user has not
explicitly given them. As an example of both cases:
containerd_src.var.in.sh (sets containerd_src variable) contains
${containerd_src:-$GOPATH/src/github.com/containerd/containerd}