Add password-only option

[EtienneM]

The different scenarios work:

scalingo login --password-only
-----> Currently unauthenticated
       Trying login with user/password:

       Username or email: customer1
       Password: 

-----> Hello customer1, nice to see you!

scalingo login
-----> Currently unauthenticated
       Trying login with SSH…
 !     SSH connection failed.
       Trying login with user/password:

       Username or email: customer1
       Password: 

-----> Hello customer1, nice to see you!

scalingo login --ssh
-----> Currently unauthenticated
       Trying login with SSH…
 !     SSH connection failed.
 !     An error occured:
       fail to login with SSH: fail to create API token: request failed: server internal error - our team has been notified

Fix #351