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
diff --git a/src/com/mirantis/mk/Salt.groovy b/src/com/mirantis/mk/Salt.groovy
index 87f7c8a..71657d6 100644
--- a/src/com/mirantis/mk/Salt.groovy
+++ b/src/com/mirantis/mk/Salt.groovy
@@ -627,7 +627,7 @@
 
 def setSaltOverrides(master, salt_overrides, reclass_dir="/srv/salt/reclass") {
     def common = new com.mirantis.mk.Common()
-    def salt_overrides_map = readYaml(salt_overrides)
+    def salt_overrides_map = readYaml text: salt_overrides
     for (entry in common.entries(salt_overrides_map)) {
          def key = entry[0]
          def value = entry[1]