Description of problem:
> For RHEL6 hosts, output of "/api/hosts?full_result=true"  shows null value for the 'uptime_seconds' field

Version-Release number of selected component (if applicable):
> Satellite 6.9

How reproducible:
> Always

Steps to Reproduce:
1. Register a RHEL 6 host to satellite or use an existing RHEL 6 host.
2. Run the following command.

# curl -s -k -u admin -X GET 'https://localhost/api/hosts/<host id>?full_result=true' |json_reformat | grep -i uptime

Actual results:

For RHEL6 System -
# curl -s -k -u admin -X GET 'https://localhost/api/hosts/7?full_result=true' |json_reformat | grep -i uptime
    "uptime_seconds": null,

For RHEL 7 system -
# curl -s -k -u admin -X GET 'https://localhost/api/hosts/4?full_result=true' |json_reformat | grep -i uptime
    "uptime_seconds": 1222722,


Expected results:

For RHEL6 System -
# curl -s -k -u admin -X GET 'https://localhost/api/hosts/7?full_result=true' |json_reformat | grep -i uptime
    "uptime_seconds": <Uptime value in seconds>,

Additional info: