Provide user override option on command line

Allows overriding a user on the command line from the one specified in
the docker-compose.yml

The added tests verify that a specified user overrides a default
user in the docker-compose.yml file.

Based on commit f2f01e207b491866349db7168e3d48082d7abdda by @chmouel

Signed-off-by: Ian VanSchooten <ian@badgelabsllc.com>

tests/unit/cli_test.py

@@ -120,6 +120,7 @@ class CLITestCase(unittest.TestCase):
             'SERVICE': 'service',
             'COMMAND': None,
             '-e': ['BAR=NEW', 'OTHER=THREE'],
+            '--user': None,
             '--no-deps': None,
             '--allow-insecure-ssl': None,
             '-d': True,