[IN_PROGRESS] - P4CredentialsProvider extension point

This is a draft implementation of <code>P4CredentialsProvider</code> extension point, which allows to use various credential sources.

The implementation supports following features "our-of-the-box":
* Local credentials definition (the legacy behavior)
* Credentials from the global configurations

Such approach allows to implement integration with [Credentials plugin][1] by an external plugin with a greater core version dependency.

The main issue is a conflict with the current validation routines, which require credentials as well. The implementation also needs some refactoring and documentation.

[1]: https://wiki.jenkins-ci.org/display/JENKINS/Credentials+Plugin