renamed get_all_vmtypes method to get_all_instance_types to better match...

... actual DescribeInstanceTypes call euca switched to.

I've been meaning to update this call for a while since eucalyptus changed from DescribeVmTypes to DescribeInstanceTypes right after the initial pull request for this code. Recently, the euca: namespace was removed from the XML response, so I also made that change. Since the old call didn't work, this new code will work properly with the forthcoming Eucalyptus 4.0 release. This call should be pretty stable in Eucalyptus now, so I don't expect further changes unless we decide to offer more detailed information beyond cores/mem/disk