Remove / Encrypt Private Keys in Database

[sam-bertin]

The private key from the SigningRequest are stored in the data base without encryption for now.
It might be a good idea to either encrypt them or remove the private key from the instance when the instance has been associated to a Certificate.

[alextreme]

Discussed with @CharString, this is a valid issue. As database encryption has other problems of its own I suggest that we remove the privatekey from the SigningRequest after a valid certificate has been uploaded