core: refactor DnsSrvLocator to allow mocking

This patch refactors DnsSrvLocator in order to allow
a mocking in a way that a list of SRV records can be
passed to the mock.

Change-Id: Ic1f04c62ff4206aed0ef5b40ff14163be7b145f3
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>