docker: os: Allow caching vuls db between builds

docker: os: Allow caching vuls db between builds

Vuls database files are quite large and take a lot of time to download,
occasionally leading to out-of-memory issues on certain Jenkins slave
build nodes.

To overcome this limitation, allow the build process to cache its data
between subsequent runs:
- if the build host machine (i.e. build server) already has a file at
  /opt/akraino/validation/tests/os/vuls/db.tar.gz,
  that file will be included in the Docker build context and extracted
  during build (Dockerfile ADD implicitly does that for .tar.gz files),
  then brought up to date and re-archived;
- to enable vuls DB caching, one can create an (initially) empty tar.gz
  file at the expected location on the host (build) machine using:
  $ make -C docker/os .init_db_cache
  This is a one time (manual) job, `make build` or equivalent calls will
  ensure the db.tar.gz file is kept up to date afterwards;

Implementation quirks:
- getting the updated db.tar.gz from the os docker image requires us to
  spawn the image as a running container (since Docker does not allow
  extracting files from the image directly), so we handle this step as
  a post-docker-build step via another double-colon `.build` make
  target;
- since we want to perform both a pre-docker-build (copying build
  server persistent db.tar.gz to the current build dir, i.e. inside the
  Docker build context) and a post-docker-build step (copying back the
  updated db.tar.gz from the os docker image to the build server
  persistent location), we leverage double-colon Makefile targets for
  `.build`:
  * pre-docker-build .build (from docker/os/Makefile before the include
  ../build.mk statement) is executed first;
  * .build from ../build.mk (common for all Docker builds in validation)
  is then normally executed, resulting in a tagged docker image;
  * post-docker-build .build (from docker/os/Makefile after the include
  statement) is executed last;

While at it, fix some related issues:
- fix build issue caused by a new (changed upstream) dependency on
  python 'cryptography' package >= 2.5 (which would require SSL and FFI
  libs inside the destionation container where pip tries to compile the
  new requirement) by adding the new requirement to
  pip-requirements.txt, so we also create a wheel for it in the build
  container;
- reduce verbosity of `go-cve-dictionary fetchnvd` to
  eliminate some non-printable characters from the job output;
- reduce verbosity of wget dot progress;
- add timestamp inside os build container to force Docker cache
  invalidation of database fetch/update step (to avoid missing upstream
  changes masked by Docker cache);
- split build container vuls database fetch into 2 separate steps
  covering tool download/installation, respectively fetch/update, so we
  leverage the Docker cache for the first step while always invalidating
  the cache for the second;
- `gost fetch` should use a number of threads comparable with the number
  of available processors;

JIRA: VAL-101

Change-Id: I41d7bec0e72c92da1596abd67e2c1306ef9ffffa
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>