Remove the Generator Constants class

Remove the Generator Constants class

Move the String constants out of a common class and into the individual
classes where they are used. This eliminates a set of Sonar code smells
relating to non-standard ordering of declarations. It simplifies the
code by removing the need for an empty private constructor in the static
class (i.e. containing static methods only).

Additionally, simplify the AaiModelGenerator interface by removing the
static factory method (which is not currently required). If multiple
implementations are needed we can use Spring annotations in the future.

Issue-ID: AAI-1242
Change-Id: Ica03b66ae2fd899977093d11d3e23dc3f3c8f194
Signed-off-by: mark.j.leonard <mark.j.leonard@gmail.com>