gluster: Refactor GetGlusterServersForImportQuery

Extracted methods related to fetching gluster peers of a given server
into a new utility class GlusterUtil

Also added few more tests in GetGlusterServersForImportQueryTest

Change-Id: Ib4620c19997c10b202ef5dc5abdbeee9f245937d
Signed-off-by: Shireesh Anjal <sanjal@redhat.com>