Expand shell vars when looking up EC2_INI_PATH in ec2.py

Currently when looking up EC2_INI_PATH in plugins/inventory/ec2.py a full and direct path must be given. Environment variables will tend to use other variables and built-ins to allow for portability and this should be expected for this variable. So, expansion of "~" and environment variables has been added to the lookup of the EC2_INI_PATH variable.