Description of problem:
         When the discovery process is canceled for any reason, the discovery_attribute_set related table is not properly saved because database transactions are not used. Unfortunately, rails validator does not allow to save the record anymore, therefore, all subsequent discovery updates fail with 422 "Validation failed: Discovery attribute set can't be blank".
        The initial discovery upload fails with 422 :ActiveRecord::RecordNotSaved: Failed to save the new associated discovery_attribute_set

Version-Release number of selected component (if applicable):
Satellite 6.6 snap 11

How reproducible:
Always

Steps to Reproduce:
1. Attempt to PXE boot the host to be discovered by satellite. Follow steps for provisioning from https://github.com/SatelliteQE/robottelo/blob/master/tests/foreman/cli/test_discoveredhost.py#L407 (Test : test_positive_provision_pxe_host_with_bios_syslinux)

Or,

1. We can POST the facts to satellite overs facts url -  https://satellite.com/api/v2/discovered_hosts/facts (Follow steps from - https://github.com/SatelliteQE/robottelo/blob/master/tests/foreman/api/test_discoveredhost.py#L100 ( Test - test_positive_upload_facts )

Actual results:
1. The discovered host stucks when attribute stuck is missing
2. Throws error - 'Validation failed: Discovery attribute set can't be blank'

Expected results:
Discovered Host is successfully discovered.

Additional info: