Put the developer's public SSH key in the OpenNebula VM authorized_keys file

1. The developer scheduling the build has his key in Gerrit
2. OpenNebula already asks Coin for contextualization information, and the key could be provided here
3. OpenNebula adds this key to context.sh
4. coin-agent does its magic and writes the authorized_keys file appropriately (since no cloud-init is used)

And voila, the user can log in without password! This would come really handy in combination with a resolution to QTQAINFRA-2462 so that we could just copy-paste the IP address.