Check skew more frequently

There have been a few alerts show up on clock skew, but by the time of
checking them they are resolved. We may also need to tune the warning /
critical threshold, but this change means minor clock skew shouldnt
end up with critical production alerts for 1 hour after the case (until
the check runs again)
Merge pull request #579 from alphagov/check-ntp-more-frequently

Check skew more frequently