store encrypted ssh key in DB

this MR stores a (encrypted) ssh key in the configDB.

⚠️ This must be merged after tsm-frontend!90 (merged)

The setup_crontab service, currently still listens on deprecated thing-creation message and needs some more adjustments to read the sshkey from the configDB instead of reading it from a file in a volumen.

This must be addressed in another MR.