Add a way to enable/disable credentials in a workflow

Currently, in a Kerberos cluster, you can use the <credentials> section to tell Oozie to get delegation tokens for HCat/Metastore, HS2, HBase, etc. However, this is defined in the workflow.xml, which means that Oozie will always try to get those tokens, even in an non-secure cluster, where it will likely fail. We should add a mechanism to enable/disable getting credentials so that the same workflow.xml can be used in both a secure and non-secure environment; as it is now, you have to maintain two copies of the workflow.xml.

We can do this fairly simply by adding a job-level property (e.g. oozie.credentials.skip=true) that would skip getting delegation tokens.