ewaol-config/kas-ci-build: Improve the DockerEngine class

- Simplify the DockerEngine API by removing mnt_and_env_format() and
mnt_and_env().
- Normalise arguments API using add_ prefix
- Use docstrings instead of hash comments
- Remove the intermediary True/False return value in run()

Issue-Id: SCM-2619
Signed-off-by: Filipe Rinaldi <filipe.rinaldi@arm.com>
Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Change-Id: Ie751a8415efa30a3be81f0ef137ee28d3036f0c8