Allow command options to be set from environment variables

Extend the Command base class to allow options to be set from values
in environment variables, if the user has not given the option on the
command line and the environment variable is set.

Derived classes of Command can override the implementation of the method
_GetEnvironmentOptions to configure which of its options may be set from
environment variables.

Change-Id: I7c780bcf9644d6567893d9930984c054bce7351e

main.py

@@ -120,6 +120,7 @@ class _Repo(object):
       return 1
 
     copts, cargs = cmd.OptionParser.parse_args(argv)
+    copts = cmd.ReadEnvironmentOptions(copts)
 
     if not gopts.no_pager and not isinstance(cmd, InteractiveCommand):
       config = cmd.manifest.globalConfig