GitLab

DEFAULT_TAG and CPU_REQUEST are being used to help with internal Gitlab
pipeline setups know which type of machines to run on, but has no value
outside of Arm Corp.  Gitlab CI allows for variables to be overridden
by default.  So, we can give it a default value of NULL/empty and have
everything work internally and externally by default.
Signed-off-by: Jon Mason <jon.mason@arm.com>