[JENKINS-16234] Fixed Clearcase Base support

Re-implement ViewStorage support by defining an ExtensionPoint.

Allows to define a default ViewStorage value in global settings,
which can be overriden at job level.
Renamed ColocatedViewStorage to DefaultViewStorage for clarity.

Factor the viewStorage view between ClearcaseSCM and ClearcaseUcmSCM
Help page for global settings
Merge pull request #19 from jenkinsci/JENKINS-16234

[Jenkins 16234] Support no argument for View Location in mkview