Allow EC2/IAM endpoint configuration

This commit allows configuration via environment variables as well as compute_options provided in
a recipe.

Examples:
with_driver 'fog:AWS', :compute_options => { :aws_access_key_id => 'YYYYYYYYYYYYYYYYY',
                                             :aws_secret_access_key => 'XXXXXXXXXXXXXXXXXXXXXXX',
                                             :ec2_endpoint => 'http://10.111.1.136:8773/services/compute/',
                                             :iam_endpoint => 'http://10.111.1.136:8773/services/Euare/'}

Or use the following environment variables:
EC2_URL
AWS_IAM_URL
I can't figure out who uses host/scheme/port/path ... what are they used for?  (It would be good to name them iam_*, too.)