JENKINS-20426:If IdentityManagementService is not available on server,
the fake implementation is used.
Rename to LegacyIdentityManagementService and add note about purpose.
Merge pull request #23 from jenkinsci/JENKINS-20426

[JENKINS-20426] Support TFS 2008
This is done through a fall-back implementation of `IIdentityManagementService` that does not require TFS 2010 but also [currently] can't resolve a user's display name or e-mail address.