Fix setSaltOverrides method

Currently setSaltOverrides method fails with error:
java.lang.IllegalArgumentException: Expected named arguments

as official docs states [1] readYaml method should be called, like this:
readYaml text: yaml_string.

[1]https://jenkins.io/doc/pipeline/steps/pipeline-utility-steps/#code-readyaml-code-read-yaml-from-files-in-the-workspace-or-text

Change-Id: I6baaa462669f1094d53a0367fdf62a88216030e9
Related-Prod: OSCORE-430
1 file changed