override placement for pruned node snapshot

[baabeetaa]

Problem:
atm, snapshot service is placed on swarm node having label cosmosia.snapshot.pruned==true.
In some case, eg software bug or very high disk io chain, we want it to run on specific swarm node instead.

Fix:
Use label to override. example:
adding label cosmosia.snapshot.sei=true to swarm node cosmosia38
=> run snapshot service for sei on swarm node cosmosia38