Count related non-root administration accounts into phonehome

[Tbaile]

Makes sure that we're able to take decisions with data, see how many machines have additional administrative accounts instead of just root.

[Tbaile]

QA: check with latest image if the phonehome command returns a rpcd_users entry inside the facts.

Additional checks:

• have a controller hooked up to the unit, see that the counter doesn't change
• create and delete a few admin users to see the counter go up or down

[gsanchietti]

Verified:

root@NethSec:~# phonehome | jq .facts.features.rpcd_users
{
  "count": 1
}