Bug 1411-4: MDSAL Binding2 Generator Impl

- BindingGeneratorImpl decomposition proposal
- introduction for Twirl templates for Yang Snippets generation
- rather more templates, less logic, more content
- JUnit test - to be finished
- added several twirl templates for YANG
- review comments fixed
- apply https://git.opendaylight.org/gerrit/#/c/44939/ for spec v2

Change-Id: I79cb7e9c0d41c23408949dd5d4d40d572cc86652
Signed-off-by: Martin Ciglan <mciglan@cisco.com>
Signed-off-by: Filip Gregor <fgregor@cisco.com>
(cherry picked from commit a5e2366bfdf89f9b859b0a7ecfe4bdf4b4cc423e)