re-apply ec2.py fixes for inifile

Re-enable set by command-line option and environment file.  We need this
at Stanford so we can get private DNS names to use with VPC's.

The only interesting change is moving the option parsing up a bit.
Merge pull request #324 from edx/sef/add-inifile-to-ec2.py-again

re-apply ec2.py fixes for inifile