Support of ignoreErrors for commands

cfn-init will now fail immediately if a command with the key
ignoreErrors='false' or without that key fails (eg it returns an exit
code other than 0). This is similar to what the AWS cfn-init script is
doing.

Change-Id: I41bfa36154fa8b16541a6abb489495739b772376
Closes-Bug: #1269476
Merge "Support of ignoreErrors for commands"