export slave remote fs for json api (#3206)

* export slave remove fs for json api

* Check permission for getAbsoluteRemotePath api

* Add @Restricted annotation

* Modify slaveComputer permission check

* Add javadoc

* Revert for squashed imports

* Add unit test for method 'getAbsoluteRemotePath'

* Change assert to Junit.Assert

* Add webClient test case

* Fix code style

* Fix javadoc

* Use more canonical way to test

* Remove super interface Serializable

* Change to way to determine whether ths json value is null