Fix sqlalchemy queue component partition_id

[jpbalarini]

I found that the partition id was always set to 0 when disabling the QUEUE_HOSTNAME_PARTITIONING setting, using a sqlalchemy backend. The problem was that the schedule method on the Queue component was always using the hostname as the partitioning key (independent of the QUEUE_HOSTNAME_PARTITIONING setting).

Also added a quick description of the QUEUE_HOSTNAME_PARTITIONING setting for the documentation (since it was missing).

Thanks!

[icapurro]

Thanks for this fix @jpbalarini !

[jpbalarini]

@sibiryakov what do you think? Thanks!

[jpbalarini]

@sibiryakov what do you think of these changes? Adding them will fix the case when someone disables hostname partitioning (my case)