Allow !override to be used with alias.environment

[JonathonReinhart]

This is a follow-up from #177 (comment).

#177 added a new !override YAML tag to cause an alias-level docker_args node to override the top-level docker_args, rather than the default behavior of concatenation.

Currently, an alias-level environment will be merged with the top-level environment (overriding if conflicting keys exist). This issue proposes allowing !override to be used in alias.environment also.

However, considering that environment is a dict/mapping, but docker_args is a string (which is shlexed into a list), environment kind of already naturally supports this operation. Since the environment dictionaries are merged (in order of command-line, top-level, alias), keys already override, allowing an alias to "kill-off" a top-level env var. It seems unlikely that you would want/need to kill-off all top-level env vars from an alias.