Trim leading and trailing space from nifi.properties values

Many users have complained about the difficulty of diagnosing unexpected behavior when a nifi.properties value does not match the desired value perfectly. Many property accessors perform an exact string match, and leading/trailing spaces can cause this to fail silently. This is especially to diagnose because it is often not apparent in the text editors used to display the properties file or log messages.

The property value should have both leading and trailing space trimmed before being consumed.