This change enables using diskimage-builder elements with secret securely.
For example, a rhel diskimage needs a REG_PASSWORD that could be define in
the secure file like so:
diskimages:
- name: rhel-7
env-vars:
REG_PASSWORD: secret-password
Change-Id: I814318ae0b5c9e4665f3fa3f011d8a687b540fac
Tristan Cacqueray