cleaning up 01-copy-manifests-dir

This change adds extra tests to the
elements/manifests/cleanup.d/01-copy-manifests-dir script to prevent it
from exiting with an error due to non-existant files.

The premature exit condition only seems to happen when an image is being
created without including the "base" element.

Changes
* adding file detection tests for dib_arguments and dib_environment
files

Closes-Bug: #1308224
Change-Id: Iaa8bafb6765241408b9f02d4896913d15f2f401d
Merge "cleaning up 01-copy-manifests-dir"