Add execution table to link to mistral

This is to keep track of the mistral executions that
are done for solum on each git push. This allows the
UI to be able to get detailed information on each pipeline
exection.

https://blueprints.launchpad.net/solum/+spec/pipeline
Change-Id: I10d3e6b911dd5e18973c5b5f9bfe83d88352df2b