Allow creating basic tier VMs

To allow creating Basic tier VM instances, I changed the parsing of the vm size argument passed to "vm create", to allow specifying basic_a0, basic_a1, basic_a2, basic_a3 and basic_a4.

I also allowed using a0, a1, a2, a3 and a4 as synonyms for extrasmall, small, medium, large and extralarge.