Fix#69 - A user may provide an `enable` attribute of a unit in their
cloud config document. If true, coreos-cloudinit will instruct systemd
to enable the associated unit. If false, the unit will not be enabled.
Fix#71 - The default enable behavior has been changed from on to off.
When using openssh without pam it checks for a ! prefix in the password
field, locking the account entirely if found. The other common lock
character, *, is allowed by ssh to login via ssh keys so use it instead.
