fix(bare-metal): First pass at updating PXE and install instructions.

Beyond the basic updates to the new images a few changes include:

 - Removed PXE instructions from the install-to-disk instructions.
 - Removed instructions to email Carly.
 - Document using cloud config with coreos-install.
feat(installing-to-disk): Document all coreos-install options
Merge pull request #168 from marineam/pxe

fix(bare-metal): First pass at updating PXE and install instructions.