Add ability to write values from file

Currently when running through tox artefact meta app
works incorrectly with multiline/json values, to fix this
it is possible to supply values from file.
Flag values from file allows to use temporary files,
to read values from.

Related-Prod: https://mirantis.jira.com/browse/PRODX-3921
Change-Id: I8f0198cc164ffac614767a1991a229f98b88a430
1 file changed