Add chrony (NTP) support for SLE 15.

Since SUSE Linux Enterprise Server 15, chrony is the default
implementation of NTP (
https://documentation.suse.com/sles/15-SP6/html/SLES-all/cha-ntp.html
).

This commit adds support for using chrony instead of ntp in SLES
versions >= 15.

See merge request nexedi/slapos.package!202