Make no asumption about user existence.

This way we can invoke the build
route using a username:token combination in the URL.

This is a known caveat in the Github Oauth plugin.

Refs:

https://issues.jenkins-ci.org/browse/JENKINS-21882
https://github.com/sorenh/github-oauth-plugin/commit/07247a1d4535e02d3f6e10fbb5a5485e1e8c1599
https://github.com/jenkinsci/github-oauth-plugin/pull/23
https://github.com/jenkinsci/bitbucket-oauth-plugin/blob/master/src/main/java/org/jenkinsci/plugins/BitbucketSecurityRealm.java