Added execution get method to mistral-dashboard api
Added execution get method to mistral-dashboard api

* added execution get method to mistral-dashboard api

Partially implements blueprint: mistral-dashboard-executions-screen

Change-Id: I290558b2e44f9cdcff43596b62d88341009a8730

openstack/mistral-dashboard,openstack/mistral-dashboard,openstack/mistral-dashboard