better update facts

[gillos]

Is your feature request related to a problem? Please describe.
When issuing update facts, like:

 curl https://localhost/api/v1.0/device_update_facts -d '{"hostname": "eosdist1"}' -X POST -H "Content-Type: application/json"

the ztp_mac field is left untouched. This causes ztp to not work if you like to resue a switch where the value of ztp_mac is on the wrong switch.
Describe the solution you'd like
Please update the field ztp_mac when doing a device_update_facts