gluster: fix import cluster when one of the peers is unreachable

Remove the server entries from the 'add server list' (serverFingerPrintMap)
which are unresolvable or unreachable by the engine.

Change-Id: I3e31006072ebf771ce49a73238a4fb933a6269f7
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=983051
Signed-off-by: Timothy Asir <tjeyasin@redhat.com>