Multi-version PE installation

Can now install different versions of PE on different nodes

- each node file in hosts config can have pe_dir setting
- defaults to old ENV['pe_dir']
- no longer need to pass any arguments/options to install_pe
- to upgrade simply call upgrade_pe(path/to/upgrade/dists)
  * since the info about the installed host is already set in the host config not more need for a :from setting
  * version to upgrade to is determined by the options parsing code