Added validation of client workspace name (via pattern from global configuration)

Hello Rob, 

I've added validation of client workspace according to [JENKINS-18378][1].
If you agree with such approach, I'll add "Prohibit creation of invalid workspaces" option, which will fail creation of new workspaces, which violate naming policy. By default, all validations will be disabled in order to preserve backward compatibility.

Several screenshots:
![globalconfig](https://f.cloud.github.com/assets/3000480/664492/7df4c418-d786-11e2-8f3e-cf86c5ba05ae.png)
![validationexample](https://f.cloud.github.com/assets/3000480/664504/b970bc04-d786-11e2-9d1b-e105f146bd77.png)

Best regards,
Oleg Nenashev
R&D Engineer, Synopsys Inc.
www.synopsys.com

[1]: https://issues.jenkins-ci.org/browse/JENKINS-18378