Workaround for non-UTC indexes

Indexes of elasticsearch are supposed to be in UTC, but at least, rsyslog
can only use local time to generate index name.

Kibana first finds indexes to query based on UTC time. So for east timezone,
there are a few hours, kibana will miss new created index(es) and no new data
is displayed during these hours.

This patch workarounds this issue. In most cases, there should be no indexes
for future time, so this just works without any harm. Even if indexes for
future time do exist, the real search query will limit time range before now.