Fix 'pxe_preserve_ephemeral' parameter leakage

Add 'pxe_preserve_ephemeral' to the list of parameters to be removed
from the node when tearing it down to prevent it from leaking from one
deploy to another.

Closes-Bug: #1317527
Change-Id: I83811126ec72ce46290fb4828a05ef0bbc2c2fd2
Merge "Fix 'pxe_preserve_ephemeral' parameter leakage"