(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
We split on semicolons here but only recompose on colons above... what's up with that?

This will probably also fail if you managed to get a semicolon in a path component. This is totally valid on a POSIX system:

    mkdir ';'
    PATH=$PATH:';'