Add stack-update argument for reset parameters

A new argument named clear-parameter for stack-update
will be supported by the heat engine to reset the value of the
parameter back to the default value. This argument can be
specified multiple times.

Partially-implements: blueprint troubleshooting-low-level-control
Partial-Bug: 1224828
Change-Id: Ib04dbc13978fdefd1bf3485f2654e139a3f1df8d
Merge "Add stack-update argument for reset parameters"