Credentials Plugin integration to allow credential injection and fine grained credential managment

This pull requests offers the following enhancements:

* Allow external CredentialProviders to inject credentials
* Allow users to use different api keys (users with more than one account at Blazemeter) at Job level or Folder level set global(default) key
* Manage credentials from one place (Manage Credentials)

It uses Jenkins credentials plugin to achieve all of the above features.

This enhancements will make it easy and reliable mechanism for CloudBees to inject credentials (received from Blazemeter CloudBees service provider) in to the provisioned Blazemeter Jenkins plugin on DEV@cloud.