Improve tasks statuses mapping to groups/colors in Executions widget

This issue is continuation of .

Task states are defined in https://github.com/cloudify-cosmo/cloudify-common/blob/master/cloudify/constants.py#L107-L118

The following ideas should be considered:

1. How to distinguish between pending states?
   Problem description taken from e-mail conversation, authored by ):
   Note that it seems like we are using the term pending in two contexts:
   1. The initial state – while we are in this state the node should probably be transparent.
   2. Waiting for a re-run after first execution did not succeed. While we are in this state I think we should keep the yellow color. Many tasks are planned to take a few re-runs by design. I think we should mark the task as red only if it failed all reruns.Not sure how easy / complicated #2 is. We can discuss.

2. There can be more states potentially - killed state - is considered to be introduced. Consult with backend team.