Accept new format of SSH keys

[Soulou]

Fixes #389

[Soulou]

└> scalingo -a db db-tunnel -i test  MONGO_URL
Building tunnel to 9b9dfa52-a534-4fd8-b039-966eff46c6c5.db-mongo.dbs.scalingo.com:30696
 !     An error occured:
       fail to connect to SSH server: Invalid SSH key or password: ssh: unhandled key type

└> ./scalingo -a db db-tunnel -i test  MONGO_URL
Building tunnel to 9b9dfa52-a534-4fd8-b039-966eff46c6c5.db-mongo.dbs.scalingo.com:30696
 !     An error occured:
       please use the flag '-i /path/to/private/key' to specify your private key: No authentication method has succeeded

The SSH key is correctly taken into account here

[EtienneM]

@Soulou Oups I merged but it would be nice to add a word in the CHANGELOG