VM nodes HW specification moved into variables

Changes the way of setting HW specification of VM nodes. Instead of
passing cpu, memory and disk values to create-nodes and setup-baremetal
scripts, scripts expect preset variables NODE_CPU, NODE_MEM, NODE_DISK and
NODE_ARCH. Architecture is newely added parameter - this patch removes
hardcoded i386 architecture in the scripts.

Also updates devtest.md with 64bit related instructions:
- suggests more memory and storage caused by increased memory footprint
- adds substitution of i386 arch in config/template files

Change-Id: Ie86fecf57a187a284833acab653fe1596725cdf9
Fixes: bug #1216864
Fixes: bug #1216814