(QENG-526) A friendly way for Beaker to set environment...

... variables for SUTs

- add support for option :host_env, it can be set in the host
  configuration file
- requires updating each host so that sshd allows remotely setting
  environment variables
- add host.add_env_var(key,val) and host.delete_env_var(key,val) for
  using in test writing