Adding job execution status constants

To help standardize using job statuses across modules this patch
introduces a set of constants in sahara.utils.edp for the statuses
currently in use.

Changes
* add job status constants for DONEWITHERROR, FAILED, KILLED,
PENDING, RUNNING, and SUCCEEDED
* add a list constant for the terminated statuses
* update references from string variables to constants

Partial-implements: blueprint edp-swift-trust-authentication
Change-Id: Ib0c47a5c002e135f2e2eed0a9066144c830926b3
Merge "Adding job execution status constants"