Shell-In-A-Box plugin: refactoring and renaming

* Using 'hostname' property instead of 'name'.
* Refactoring 'getShellInABoxUrl' method signature
  to get the selected host.
* Renaming all occurrences of 'Shell Box' to 'Shell In A Box'.
* Removing 'isAccessible' method.

Change-Id: Id24155f0480e0af5497c2b74eee203dfdf60e3f8
Signed-off-by: Daniel Erez <derez@redhat.com>