gnt-cluster verify: Warn if node time diverges too far

The warning will be generated if the clocks diverge by more
than 150 seconds. Due to the way the RPC system works, we
cannot get exact time differences, e.g. if one of the
queried nodes is broken. The comparision is done using a
time window.

Confd queries will fail if the clock on the client and server
are more than 300 seconds from each other. This check helps
keeping at least the nodes of a cluster in sync.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>