Use utf-8 encoding when writing config scripts

Change from default encoding 'ascii' to 'utf-8' encoding when writing
config scripts (stored in Heat software config's 'config' attribute).
The default ascii encoding causes issues when config scripts include
special characters.

Change-Id: I6f9135ad646ec4996c1c012ff11f7929404ecc3e
Closes-Bug: #1382031
Merge "Use utf-8 encoding when writing config scripts"