(PDB-675) Stop pdb process even if pidfile missing

Prior to this commit, there were issues about the init.d script failling
silently if the PIDfile was missing. Although the fact that the PIDfile
was missing is a greater issue, this at least provides a workaround for
that. When trying to stop the puppetdb service with a missing PIDfile,
the stop would fail silently, assuming the process had already been
stopped because of the missing PIDfile. However, running `ps auwx | grep
puppetdb` would show that the process has in fact, not been stopped.
This change updated the init.d script to check if the PIDfile is
present before atttempting a stop. If it is, it uses that information to
stop the process as it always has. If it isn't, then it uses the
puppetdb user to stop the process. It is likely that this is too heavy a
hammer to user for this particular problem, but I think it should do
just what we need it do. We still specify --exec when searching for the
process that needs to be stopped, which should limit the possibility of
stopping a process we actually don't want to stop.
(maint) Restrict which process will be stopped

This is kind of an after-thought, but we figured since this file is
already being edited, this would be a good opportunity to add in this
additional failsafe. The idea is that no puppetdb process should be
started by anyone except the puppetdb user. If, for some reason, the
puppetdb process fails, or is stopped, but its PIDfile isn't removed,
there is the possibility that some other process can take its old PID.
In that case, stopping the puppetdb service would look at the PIDfile,
and stop the wrong process because it had taken on the old puppetdb PID.
Granted, this situation has a very small chance of occuring, especially
since we also limit what process will be stopped with the `--exec` flag.
However, it's nice to safeguard against the possibility anyway.
(maint) Add log entry for when process stopped without PIDfile
Merge pull request #1001 from melissaanne/pdb-675/stable/missing-pidfile

(PDB-675) Stop pdb process even if pidfile missing