Migrate linter to use golangci-lint

This currently uses gometalinter which has differences in performance
than golangci-lint. It 'calls the linters directly and reuses work by
parsing program once'. From initial investigation it *is* faster, tried
on various projects but looks like they also need to do quite a bit more
work with program load times as that's still where the bottleneck it's
the majority of the time spent.

===

Don't directly replace gometalinter.

Following feedback I have re-introduced gometalinter as the default so
it's as backwards compatible as possible. I've also switched
golangci-lint to install using the script recommended by the author,
tied to the latest version which can be controlled by the environment
variable $GOLANGCI_LINT_VERSION.

===

Revert lint script

We don't want any of the changes I made to the lint script until I get
some feedback around whether we should allow the golangci-lint to be
used via flags or whether it should just be run manually, or part of a
different script.

Signed-off-by: Chris Mills <millscj01@gmail.com>

Author: Christopher Mills

rootfs/Dockerfile

@@ -10,6 +10,7 @@ ENV AZCLI_VERSION=2.0.46 \
     KUBECTL_VERSION=v1.9.6 \
     SHELLCHECK_VERSION=v0.4.6 \
     ETCDCTL_VERSION=v3.1.8 \
+    GOLANGCI_LINT_VERSION=v1.10.2 \
     PATH=$PATH:/usr/local/go/bin:/go/bin:/usr/local/bin/docker \
     GOPATH=/go
 
@@ -89,6 +90,7 @@ RUN \
     gopkg.in/alecthomas/gometalinter.v2 \
   && ln -s ${GOPATH}/bin/gometalinter.v2 ${GOPATH}/bin/gometalinter \
   && gometalinter.v2 --install \
+  && curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | sh -s -- -b $GOPATH/bin $GOLANGCI_LINT_VERSION \
   && pip install --disable-pip-version-check --no-cache-dir azure-cli==${AZCLI_VERSION} shyaml \
   && apt-get purge --auto-remove -y libffi-dev python-dev python-pip \
   && apt-get autoremove -y \