Refactor PXE clean up tests

Make this tests really unit test, leave a separate "full flow"
test for checking the most common case without mocks.
Also add missing test for iscsi_deploy.

Change-Id: Idc85efce2d58e41642a4a72c612df4a5f9f12082
Closes-Bug: #1370487
Merge "Refactor PXE clean up tests"