add new topology parameter: arch

if unset, arch variable will default to "x86_64"

* Note on "arm64" vs "aarch64"
  debian-based uses arm64
  rhel-based uses aarch64
  qemu binaries of both distribs uses aarch64
  dpdk uses arm64
  vpp uses aarch64
  python machine modules uses aarch64
=> prefer aarch64 to use the same nomenclature as vpp

* add ARCH argument to:
  init_dpdk.sh, install_dpdk.sh, run_l2fwd.sh, install_tldk.sh, run_tldk.sh.
  default to x86_64
  converts "aarch64" if needed for dpdk naming convention
* fixed terminal end detection to allow "~]# "
  add dut node arch as param to all robot set bin calls
* add --target-list flag to qemu_build.sh
  defaults to x86_64-softmmu
* add arch flag to all the topology files
  * topologies/available/ (and example file)
  * resources/tools/virl/topologies/
* set _qemu_bin path using node['arch'] in qemu_set_node()

Change-Id: If46d88d064d213d3e4c6fc584bb8e0d4b6428cb8
Signed-off-by: Gabriel Ganne <gabriel.ganne@enea.com>