Commandline client environment variable setting and logging updates.

* Adds update_environment_variables() to update internal os_env_var
  dictionary after instantiation.
* Adds run_async_command() method to run a command and immediately
  return a process object instead of waiting for the process output.
* Modifies run_command to use run_async_command().
* For each request, logs the intersection of all environment
  variables that are present in the environment and that have been
  set in the environment by the client.
* Cleans up log line formatting.

Change-Id: I845bfaf9de096fbfe523722e5a21c1b6dde76363
Merge "Commandline client environment variable setting and logging updates."