Fix Cassandra service status definition

Reasons:
 - At slow environment (small flavors, etc.),
   for certain reasons Cassandra instances are in SHUTDOWN state
   for short period of time. This issue breaks assertion
   in int-test that waits for guest to reach ACTIVE state.

Changes:
 - timeout at start_db added;
 - polling mechanism added;
 - tests;

Change-Id: If50c1a2fd8778e2472a1266817a42d5c3c73d8b6
Closes-Bug: #1314980