bug 857100 - reset time by seconds and microseconds too

@rhelmer r?

From the comment  around line 146 in socorro/cron/base.py:

```
# The last_success datetime is originally based on the
# first_run. From then onwards it just adds the interval to
# it so the hour is not likely to drift from that original
# time.
# However, what can happen is that on a given day, "now" is
# LESS than the day before. This can happen because the jobs
# that are run BEFORE are variable in terms of how long it
# takes. Thus, one day, now might be "18:02" and the next day
# the it's "18:01". If this happens the original difference
# will prevent it from running the backfill again.
```

That example shows how the "resetting" of the hour and minute solves the problem of potential drift when the job is initiated the second time 1 minute earlier than the day before. 

However, what happened in https://bugzilla.mozilla.org/show_bug.cgi?id=857100#c8 (I think) is that it started 7 seconds earlier than normal. 

By removing the seconds and microseconds entirely we avoid this problem when under 1 minute.