wip for RHEL7 support in base.install
RH7.0/build/repositories: use subscribe-manager tool

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
build/repositories: use --force with subscribe-manager

Using --force ensures us that even if we already subscribed, it won't
return a error code and won't make the build fail.
build/repositories: fix "case" in adding missing ;;
build: add support for CentOS 7
build: create the CODENAME_MAJOR and CODENAME_MINOR variables and use them.
Merge remote-tracking branch 'origin/RHEL7'

* origin/RHEL7:
  build: create the CODENAME_MAJOR and CODENAME_MINOR variables and use them.
  build: add support for CentOS 7
  build/repositories: fix "case" in adding missing ;;
  build/repositories: use --force with subscribe-manager
  RH7.0/build/repositories: use subscribe-manager tool
  wip for RHEL7 support in base.install
build/repositories: use the same way as in other functions to get the
major part of the release. To do so in base.install, fake this major
variable to have the same interface.