Web: stop polling when master status is "stop"

Some folks are bandwidth-limited or data-capped.  Right now poudriere
polls forever, even when the bulk run is complete.  After this commit, it
will stop polling when the bulk run is complete.  The .data.json files
can get to be huge (> 1.5M uncompressed).

If there is a negative, it's that a "latest link" won't automatically
pick up a new bulk run.  In reality, this would show erroneous information
until new values for each status type had at least one element.  In any
case, if I have to choose, I prefer to minimize internet usage.