Generate-repo context: remove repo-context.yml if exists explicitly

Base method writeYaml failed in case of file repo-context.yml exists
and can't automatically override it, so let's remove file if exists.

Change-Id: I56cfc22608ccb17b095cbd186181d016b837e9e2
diff --git a/generate-repo-snapshot-context.groovy b/generate-repo-snapshot-context.groovy
index e51dbc4..c36aeae 100644
--- a/generate-repo-snapshot-context.groovy
+++ b/generate-repo-snapshot-context.groovy
@@ -69,6 +69,8 @@
             }
         }
 
+        // remove file if exists
+        sh "rm -rf ${fileName}"
         writeYaml file: fileName, data: ['parameters': meta ]
         archiveArtifacts artifacts: fileName
     }