Allow Pull Secret and SSH Keys to be Specified by the end user

[sabre1041]

Currently, the location for the SSH and OpenShift Pull Secret that is populated into the install-config.yaml is hardcoded to the following locations:

• Pull Secret: $HOME/pull-secret.json
• SSH Key: $HOME/.ssh/id_rsa

Allow the end user to specify the location that these assets should be retrieved from

[butler54]

Why don't we just use an optional enviornmental var override? Easiest way to be backwards compatible?