webadmin: fixed generics in EnumTranslator, fixed typo

Parameterized EnumTranslator could accept any type, not just Enums,
which is wrong. Constructor accepted also any type, but it was
expected (not reinforced) that this type will be
Class<? extends Enum<?>>.

Change-Id: If835e719a497e7ab1db4fc4f9484354e838c60c5
Signed-off-by: Martin Mucha <mmucha@redhat.com>