Remove image caching from 'prepare'

Today we try to cache images when 'prepare' is called. It turns out
this doesn't quite match Ironic's model, where 'prepare' is more about
managing the local "deployment environment" which we don't really use.

Eventually we'll want to implement image caching as a vendor passthru
method.
Remove an extraneous node save in 'tear_down'

Right now 'tear_down' includes an unecessary save of the node. Remove
this and add a TODO regarding decom.
support vendor passthru
Merge pull request #17 from rackerlabs/support-vendor-passthru

support vendor passthru
Actually import passthru
Merge pull request #18 from rackerlabs/support-vendor-passthru

Actually import passthru
Switching to more specific exceptions
Stylistic fixes
Merge pull request #19 from rackerlabs/JoshNang/specific-exceptions

Switching to more specific exceptions
fix hardware type check
Merge pull request #20 from rackerlabs/support-vendor-passthru

fix hardware type check
Merge pull request #14 from rackerlabs/no_prepare

Remove image caching from 'prepare'
Merge pull request #15 from rackerlabs/remove_duplicate_save_in_tear_down

Remove an extraneous node save in 'tear_down'
Expose our vendor interface on TeethDriver

We have a vendor interface already, but it isn't registered on
TeethDriver. This also renames 'teeth_ironic_driver.passthru' to
'teeth_ironic_driver.vendor', and TeethVendorPassthru to
TeethVendorInterface.