CI: Allow job tag to be optionally set globally

CI: Allow job tag to be optionally set globally

The default behavior of Gitlab runners is to only run jobs which match
the configured tag, although there is an option to run untagged jobs
[1].

To support running the CI in more complex environments where different
types of runners may be present that support different tags, allow the
DEFAULT_TAG for all jobs in the pipeline to be set globally using an
environment variable. An empty default value is provided to retain
support for untagged runners.

[1] https://docs.gitlab.com/ee/ci/runners/configure_runners.html#use-tags-to-control-which-jobs-a-runner-can-run

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>