customscripts: exclude decommissioning hosts from mgmt data

decom'ing hosts don't have their hostname entry in mgmt dns (asset tag entry is there though).

Therefore don't export said hosts as part of mgmt data to make sure all mgmt FQDNs are actually
resolvable in DNS.

Bug: T310266
Change-Id: I04cba8d8b333ee703950e2cbb526c8295b9a7beb