Fix Amazon Linux 2 scanning (#630)

* fix(amazon2): fix OS version parse error

config/config_test.go

@@ -62,3 +62,42 @@ func TestSyslogConfValidate(t *testing.T) {
 		}
 	}
 }
+
+func TestMajorVersion(t *testing.T) {
+	var tests = []struct {
+		in  Distro
+		out int
+	}{
+		{
+			in: Distro{
+				Family:  Amazon,
+				Release: "2 (2017.12)",
+			},
+			out: 2,
+		},
+		{
+			in: Distro{
+				Family:  Amazon,
+				Release: "2017.12",
+			},
+			out: 1,
+		},
+		{
+			in: Distro{
+				Family:  CentOS,
+				Release: "7.10",
+			},
+			out: 7,
+		},
+	}
+
+	for i, tt := range tests {
+		ver, err := tt.in.MajorVersion()
+		if err != nil {
+			t.Errorf("[%d] err occurred: %s", i, err)
+		}
+		if tt.out != ver {
+			t.Errorf("[%d] expected %d, actual %d", i, tt.out, ver)
+		}
+	}
+}