Fix JENKINS-22169

If setenv and envinject are both configured for a job the
EnvInjectMigrationBuildWrapper will overwrite existing envinject config.

This fix introduces the migration in SetEnvBuildWrapper to the original project
so it can check if there is any existing config and transfer that over before
returning the replacement. It currently only looks at the propertiesContent
and any other config will still be overwritten.

Change-Id: Ief68f06d714f0a31f8fc8461aaa4cd893c64599b
Merge pull request #28 from rsandell/jenkins-22169

Fix JENKINS-22169