Remove plugin support and use jenkins_api_client gem instead of own impl
Support ERB Template for credential id switching:(

Since Jenkins changed its API for managing node credential.
I have no idea when this changed, but for now, we need to set
`credential id` which is managed by Credential Plugin.
Unfortunately, Credential Plugin doesn't have any REST interface,
so we need to create credential by hand, and then set to config.xml.

This commit enables user to switch node credentials for different
environemnt such as production, staging and etc.
(WIP)Fix example for new configuration