Give nodepool user sudo access for dib

Disk image builder requires root permissions to chroot and mount images.
Update the puppet manifest for nodepool to optionally enable
passwordless sudo access for the nodepool user. This defaults to being
allowed but can be toggled if this is deemed an unnecessary security
risk.

Change-Id: If0bf5f182d88c848cd2a64c5c75cc64cc0b42c58