There seems to be some inconsistency in the value of parameter "compute_attributes":{"start":"1"} during host creation.
This parameter accepts only string containing number ("1") and not boolean parameter True or number 1. Which is quite inconsistent behaviour in comparison with other boolean parameters.

I'm trying it via python-foreman library from python with this host configuration:

{'build': True,
'comment': 'Testing VM test',
'compute_attributes': {'start': '1'},
'compute_profile_id': 5,
'compute_resource_id': 1,
'domain_id': 2,
'enabled': True,
'hostgroup_id': 5,
'interfaces_attributes': [{'ip': '192.168.123.2',
'mac': '',
'managed': True,
'primary': True,
'provision': True,
'type': 'interface'}],
'location_id': 1,
'managed': True,
'medium_id': 9,
'name': 'test',
'operatingsystem_id': 1,
'provision_method': 'build',
'root_pass': 'abeceda123'}

When I set compute_attributes - start to 1 (number) or True (boolean) it is ignored and the Host is not started.

Environment: RHEL 7, foreman-1.12.2-1.el7.noarch