K8s remove string ops (#2119)

* plugin/kubernetes: remove bunch a string ops

This removes a bunch of appends to where not needed, makes dnsutil.Join
take variadic args which removes the need to wrap in a new string slice.

Signed-off-by: Miek Gieben <miek@miek.nl>

* Fix calls to dnsutil.Join

Signed-off-by: Miek Gieben <miek@miek.nl>

* Revert these

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/pkg/dnsutil/join_test.go

@@ -9,11 +9,12 @@ func TestJoin(t *testing.T) {
 	}{
 		{[]string{"bla", "bliep", "example", "org"}, "bla.bliep.example.org."},
 		{[]string{"example", "."}, "example."},
+		{[]string{"example", "org."}, "example.org."}, // technically we should not be called like this.
 		{[]string{"."}, "."},
 	}
 
 	for i, tc := range tests {
-		if x := Join(tc.in); x != tc.out {
+		if x := Join(tc.in...); x != tc.out {
 			t.Errorf("Test %d, expected %s, got %s", i, tc.out, x)
 		}
 	}