build: ISO refactor, use docker, enable cache

build: ISO refactor, use docker, enable cache

Previously, Armband Makefile used to call Fuel@OPNFV Makefile
target <iso>, instead of <all>, which resulted in bypassing
using Docker for ISO building completely.

Switch to <all> Makefile target, hence bringing back Docker
as a build container.

This change justifies moving make variables to a separate spec
file, armband.mk.

While at it, enable caching of builds.
This requires Jenkins bash scripts to be updated for the ISO
build job, done in [1].

JIRA: ARMBAND-56

[1] https://gerrit.opnfv.org/gerrit/#/c/18453/

CHANGES:
- all: release (was all: build)
- Common make targets now depend on submodules-init;
- clean-build and clean-docker make targets have been replaced by:
  make clean, make deepclean;

FIXME: These issues should be fixed later:
- (optional) trim local repo paths from gitinfo files
- only account for armband submodules in cache fingerprints;
- identify size mismatch culprit

Change-Id: I7d73409be44983fa21d4c09d2e07b87a33231e03
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>