Deploy centos 6.5 as slave

This pull request adds several new things:
* Fully baked in proxy support.  All aspects of the bootstrap and compute node bringup will wind up using the same proxy as the host system or your development environment.
* Switch back from doing fully online installs of slave nodes to installation from a local copy of the OS install media.  Fully online installation was taking too long and is really bad for environments that do not have a caching web proxy in the loop.
* Readd support for CentOS 6.5 as the OS on a slave node.
* Port over the full disk reservation system from the Crowbar org.
* Modify the docker-admin script to mount `/tftpboot` from `$HOME/.cache/opencrowbar/tftpboot`.  This lets us cache some expensive package fetches and ISO extractions between docker-admin runs.