gce: Check if user logins with GCE

It's easy for new user to forget to login with GCE before running
capstan + GCE.  Do a check and prompt friendly.

If you do not login with GCE, it will prompt:

   You do not currently have an active account selected.
   Please run:

     $ gcloud auth login

   to obtain new credentials, or if you have already logged in with a
   different account:

     $ gcloud config set account <account name>

   to select an already authenticated account to use.

Signed-off-by: Asias He <asias@cloudius-systems.com>
Merge pull request #108 from asias/gce.login

gce: Check if user logins with GCE