cleanup around seemingly impossible cast

- changed type parameters on details map in CreateSecondaryStagingStoreCmd - <String, String> was misleading since it can not work with a string value and it is never a string
- introducing the type parameters allowed some simplifications in getDetails()
- added unit test

Signed-off-by: Laszlo Hornyak <laszlo.hornyak@gmail.com>
Signed-off-by: Pierre-Luc Dion <pdion891@apache.org>
CreateSecondaryStagingStoreCmdTest.java: add license header