utils: ServletUtils: support ETag for static content

Move modified ETag logic from BrandingServlet into the ServletUtils, so
we can reuse it in different implementations.

Cleanup a bit the BrandingServlet to relay on ServletUtils validations.

Change-Id: I9b6c30f1ff8f51c5370074de1186a3f7a7cfeac7
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>