Script to create cfn-* symlinks in /opt/aws/bin

Typical usage:
cfn-create-aws-symlinks --source=/usr/bin

This will create directory /opt/aws/bin if it does not exist, and then creates symlinks within that from the scripts /usr/bin/cfn-*.

By default, any existing /opt/aws/bin/cfn-* script will not be overwritten. This behaviour can be overridden with the flag --force
Move logic to a couple of functions and parse args only in __main__
Merge pull request #1 from steveb/master

Script to create cfn-* symlinks in /opt/aws/bin