Replace 'EXAMPLE' in the AWS config with environment variables

This change replaces the use of 'EXAMPLE' with references to appropriately named environment variables. I intentionally have not modified the other providers - digital ocean et al - although similar things could be done there as well.

An example of a bash script to set up the variables (these values are made up!):

    $ cat ~/.awskeys
    export AWS_KEY_ID="OIFUVB3SIT8GSNTKGSUT"
    export AWS_SECRET_KEY="9CVjK7xVBIER8N9z8rvVrkw8nvO/EANO+nI&EFbL"
    export AWS_KEYPAIR_NAME="noname"
    export AWS_KEY_PATH="~/.ssh/id_rsa"
    export AWS_SECURITY_GROUP="default"

and then use that using bash 'source' or bash/sh 'dot' syntax:

    $ source ~/.awskeys
    $ .  ~/.awskeys

Or just include them in your .bashrc or .profile files.