GitHub #557 wrong result in int8 model

 * Added support for qasymms8 (int8) to ImageTensorGenerator
 * Added qasymmu8 as alias for qasymm8 in ImageTensorGenerator
 * Added support for qasymms8 (int8) to ExecuteNetwork
 * Added qasymmu8 as alias for qasymm8 in ExecuteNetwork
 * Set tflite to be the default model format in ImageTensorGenerator as
   it's the only supported model format.

Signed-off-by: Mike Kelly <mike.kelly@arm.com>
Change-Id: Ieda7b78e668ea390e3565cd65a41fe0a9c8a5b83