Fix testtool on input modules without revision

Fix testtool on input modules without revision

- According to RFC 7950 revision number in module is not
  mandatory (only recommended) - currently Netconf Testtool fails
  on such modules with NullPointerException because it cannot find
  revision line pattern.
- Revision number doesn't have to be placed in input YANG schema -
  the filename is kept unmodified.
- Regex pattern that matches date in revision is also supplemented
  by revision keyword and optional double-apostrophes - there could
  be a collision with date in include statement.
- YANG module capability model is modified so the revision can be
  null - reurning of Optional.absent().
- DummyMonitoringService sets empty string in place of null
  revision numbers.

Change-Id: I868d39c111d788a0de86251da10f76a448f47de4
Signed-off-by: Jaroslav Tóth <jtoth@frinx.io>