Support for multiple regions in AWS

Changes:
* added cli location parameter (-L or --location)
* initialize libcloud functions with a region-specific connection object
* modified namespaced_function to set default parameter when wrapping methods from libcloudfuncs

This patch should allow the region to be specified in the following order (each overriding the former):
1. Global salt-cloud config
2. Cloud profile setting
3. CLI parameter

Only AWS is supported in this patch.