Support for dynamically generated password for Windows boxes

[fred06fr]

I'm trying to provision Windows boxes. Our OpenStack images dynamically create passwords for those on instanciation.
This makes the plugin break on provision, as winrm cannot connect to the box without the password to perform the setup.
The password can be retrieved from OpenStack using 'nova get-password'
The proposal is to add a feature in the plugin to do that automatically: if winrm password is set to a special value ':dynamic', retrieve it from OpenStack.
I'm working on a patch to do that.