Support other EC2 compliant API's

An example is eucalyptus.  By providing aws.endpoint and aws.version this patch allows vagrant-aws to deploy on a eucalyptus cluster (and presumably other EC2 compliant clusters such as openstack).

I have tested this on my Eucalyptus cluster, and it works fine.  For Eucalyptus you need to use:

endpoint = http://YOUR_EUCALYPTUS_CC:8773/services/Eucalyptus
version = "2010-08-31"