luna-sysservice: Make the run-time dependency on sntp configurable

:Release Notes:
The use of sntp by luna-sysservice is optional, so make the run-time
dependency on it be configurable via VIRTUAL-RUNTIME_ntp.

:Detailed Notes:

:Testing Performed:
"make webos-image" succeeded.

:QA Notes:

:Issues Addressed:
[GF-58977] Make the run-time dependency on sntp configurable
[GF-59277] CCC: Make SNTP dependency configurable

Open-webOS-DCO-1.0-Signed-off-by: Nikolay Orliuk <nikolay.orliuk@lge.com>
Open-webOS-DCO-1.0-Signed-off-by: Susan Montooth <susan.montooth@lge.com>

Change-Id: Ie7ac13ef370444e52d50b576ebfac821ec82ccf8
Originally-reviewed-on: https://g2g.palm.com/4468
Cherry-picked-from-commit: 2d5607ff412085188cbb424de70b97c483733263
Cherry-picked-from-branch: @acadia