CLOUDSTACK-4453: fetch host credentials from marvin config

Tests would fetch the credentials for the host to hop into router to
check for essential services. Each test would require to put in the host
information into the test data. Instead fetch the credential information
from the marvin configuration file.

Signed-off-by: Prasanna Santhanam <tsp@apache.org>
(cherry picked from commit 4b546ce85d40098ade69c575316e76e25a422a12)