Updated tools version to 2.0.0

Also merged lint-style (gofmt) into golangci-lint (enabled gofmt).
Also removed redundant lint-sanity (govet) which is already checked by golangci-lint.
Also made minor code changes to pass new golangci-lint tests.
Also changed lint-mod slightly so it reverts any changes it makes go.mod, go.sum, and vendor/.

VOL-2647

Change-Id: I2919e65ae29e01c81d1bf7ef113c3189e8227939
diff --git a/docker/Dockerfile.ofagent-go b/docker/Dockerfile.ofagent-go
index b1a6f33..6c224f5 100644
--- a/docker/Dockerfile.ofagent-go
+++ b/docker/Dockerfile.ofagent-go
@@ -15,7 +15,7 @@
 # -------------
 # Build stage
 
-FROM golang:1.12-alpine3.9 AS build-env
+FROM golang:1.13.8-alpine3.11 AS build-env
 
 WORKDIR /src/
 COPY . /src/
@@ -29,7 +29,6 @@
 
 # Build ofagent-go
 SHELL ["/bin/ash", "-o", "pipefail", "-c"]
-ENV GO111MODULE=on
 RUN go build -mod=vendor -o /build/ofagent \
 	-ldflags \
 	"-X github.com/opencord/voltha-lib-go/v2/pkg/version.version=$org_label_schema_version \
@@ -44,7 +43,7 @@
 # -------------
 # Image creation stage
 
-FROM alpine:3.9.4
+FROM alpine:3.11.3
 
 # Set the working directory
 WORKDIR /app