Expand environment variables in repository URL.

Expand environment variables in repository URL in job confguration during validation check, affecting also entering of credentials.

This should allow you to use the same `$JENKINS_HOME` contents while switching the physical location of your Subversion server using a startup environment.

(Has no effect when _Validate repository URLs up to the first variable name_ is checked.)

Also expands build variables (e.g. those defined in a containing folder property) when defining `SVN_*` variables, which would otherwise go unset.

Might solve [JENKINS-11667](https://issues.jenkins-ci.org/browse/JENKINS-11667).