system_ntp: wait for sync via kernel
system_ntp: wait for sync via kernel

We've been having more and more trouble with dist-test slave clocks becoming
desynchronized. Because slaves run inside docker containers, we can't use
ntpd/chrony-based tools such as ntp-wait to wait for synchronization.
Moreover, this highlights a general issue with the existing wait-for-sync
approach: we can't differentiate between environments where ntpd isn't
running but should be, and environments where it's not but synchronization
is still expected.

This patch switches from ntp-wait and friends to ntp_adjtime-based waiting.
Originally it also cut the wait time from 60s to 30s (as an overture for
users running in the first environment who would prefer not to wait a full
minute to know that their ntpd isn't running), but this proved to be
insufficient to ride out dist-test clock desynchronization.

I also looked at the source code for ntp-wait and chronyc, but didn't see
them doing anything fundamentally more interesting than what we're now
doing with ntp_adjtime.

Change-Id: Ic133c4f9b5fd933216fb27f9f01396a5fee8276b
Reviewed-on: http://gerrit.cloudera.org:8080/13323
Tested-by: Kudu Jenkins
Reviewed-by: Will Berkeley <c74686ab413a71926fc1bc9d16ebfa30e619336a@gmail.com>

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