Fix default user and password cloud-init issues

When cloud-init is being used, we should only attempt to set a
new default user if one was actually specified in VM_USER_ID.  The
cloud-config setting for this new default user's password was also
incorrect, which was causing VM_ROOTPW to be used instead of the
value in VM_USER_PW.  If a VM_USER_ID was not specified, we simply
use VM_ROOTPW to modify the password of the standard default user.