If not on systemd:
  assume enabled unless disabled by /etc/cloud-init.disabled or kernel cmdline.

if on systemd:
3 use cases.  considering booting in systemd.
 a.) running before the cloud-init-generator
     - not something i'm worried about right now.
 b.) ci is disabled due to ds-identify seeing nothing
     accepting we are not in 'a' above, the generator has written
    /run/cloud-init/enabled if it is enabled.
    so if this file is not there, then we are disabled.
 c.) ci is disabled via kernel command line or /etc/cloud/cloud-init.disabled
     - easy enough to re-implement/re-use the non-systemd case
       but also 'b' applies here.