Add {get,set}SchemaFolderLocation() to the public Pfw API

It will allow users to tell the parameter-framework where the Schemas are
instead of letting it guess.

These methods are also added to both "Platform" and "Full" connectors.

Change-Id: I7fe3b2fd2c6dba1d90d36de97e6db7d71b2c686f
Signed-off-by: David Wagner <david.wagner@intel.com>