node-get-console incorporate the changes in API

As of commit I6c5818052e18ddfbcc148a18976ba5873b644039 to ironic
changed the return value for the console GET api from string to
a JSON object, this patch made the corresponding changes in the
node-get-console command.

Change-Id: I4d12a966e24a3ac04a0dfbb7d1fa489b69201749
Closes-Bug: 1307555