Add _create_new_stack method

Add _create_new_stack method

The logic for creating a new stack in contexts/heat.py:HeatContext
can added to a method to make the code eassier to read and test.

This is in preparation for an update to deploy() that would allow
an existing stack to be reused, or a new stack created.
By having the create_new_stack logic in a new method, deploy()
becomes easier to read and test.

JIRA: YARDSTICK-886
Change-Id: I7af01e2209a3460658f8db0249b7c620743cced0
Signed-off-by: Emma Foley <emma.l.foley@intel.com>