e2e: enable custom kernel building, caching and installing (CXL-2)

[askervin]

- kernel_getsource environment variable specifies either a
  shorthand ("vanilla", "mainline" or "cxl-next" to start with)
  or direct shell command(s) that result in "linux" directory with
  kernel sources in it.

- kernel_config specifies kernel .config for a custom kernel filename.
  The file is located on host in playbook lookup('file', ...) path.

- Ansible playbooks make sure that custom kernel packages, should they
  be needed, are located in vm when test script execution starts. The
  test script can install and uninstall the packages at will.  If
  kernel packages for kernel_getsource and kernel_config are already
  cached on host, playbooks copy them on vm. If not, playbooks fetch
  sources on vm, applies .config, installs kernel build dependencies,
  buildds kernel packages on vm and caches them on host.