Add region parameter and changed connection code.

This is preferable for future cross-region work with the ec2 module.  It also directly addresses some recent issues numerous users have had.  These issues have involved trying to launch an instance and work with resources which exist in a different region.  In many cases, the EC2_URL environmental variable has been set incorrectly.  The region (and related zone) parameter allow the user to easily override the assumed environmental variables to make sure they are working in the zone which contains all of their resources (image, keypair, security groups etc.).  I'll look to replicate in the ec2_vol module.