* test/plugin: test external db plugin
* use test helper to get cluster and plugins
* create test helper to create a vault admin user
* add step to revoke lease
* make tests parallel and add reload test
* use more descriptive name for test group; check response
* test/plugin: refactor compilePlugin for reuse
- move compilePlugin to helper package
- make NewTestCluster use compilePlugin
* do not overwrite plugin directory in CoreConfig if set
* fix getting plugin directory path for go build