Environment names can be used in a number of contexts, and should be
properly escaped for safety. Since most state names are store in path
structures, and often in a URL, use `url.PathEscape` to check for
disallowed characters
`env list` was missing the args re-assignment after parsing the flags.
This is only a problem if the variables are automatically be populated
as arguments from a tfvars file.
Add Env and SetEnv methods to command.Meta to retrieve the current
environment name inside any command.
Make sure all calls to Backend.State contain an environment name, and
make the package compile against the update backend package.
James Bardin