spark token - only prompt for password

[kennethlimcp]

The user is already logged in so instead of having to repeatedly enter the email address each time you want to do something with regards to access token, we should simply print-out the current profile email address and prompt for password:

Current prompt:

$ spark token list
Checking with the cloud...
Could I please have an email address?  xxx@xxxxcom
and a password?  *******

Proposed prompt:

$ spark token list
Checking with the cloud...
You are logged as xxxxxx@xxxxxx.com 
Enter your password to revoke/create/list access_token : *******

[kennethlimcp]

Closing this since it seems like the code is indeed checking for a valid username and i happened to not be logged on.

Another fix to tell the user that they are not logged in here: https://github.com/spark/spark-cli/pull/177/files#diff-b3165f51cd169144b4f8b5a51c452f89