Prepare for dependency vendoring.

- Add helper script to simplify glide usage.
- Add validation script for unwanted changes to vendoring.
- Relax/tighten up .{git,docker}ignore to cover vendored files properly.
- .validate: Protect from unbound variable in case of nounset setting.
- Install more recent hg version in the build container.
- Remove glide installation steps from Dockerfile.
- Update documentation.

build.Dockerfile

@@ -1,10 +1,18 @@
 FROM golang:1.7
 
+# Install a more recent version of mercurial to avoid mismatching results
+# between glide run on a decently updated host system and the build container.
+RUN awk '$1 ~ "^deb" { $3 = $3 "-backports"; print; exit }' /etc/apt/sources.list > /etc/apt/sources.list.d/backports.list && \
+  DEBIAN_FRONTEND=noninteractive apt-get update && \
+  DEBIAN_FRONTEND=noninteractive apt-get install -t jessie-backports --yes --no-install-recommends mercurial=3.9.1-1~bpo8+1 && \
+  rm -fr /var/lib/apt/lists/
+
 RUN go get github.com/jteeuwen/go-bindata/... \
 && go get github.com/golang/lint/golint \
 && go get github.com/kisielk/errcheck \
 && go get github.com/client9/misspell/cmd/misspell \
-&& go get github.com/mattfarina/glide-hash
+&& go get github.com/mattfarina/glide-hash \
+&& go get github.com/sgotti/glide-vc
 
 # Which docker version to test on
 ARG DOCKER_VERSION=1.10.3
@@ -24,13 +32,4 @@ RUN mkdir -p /usr/local/bin \
     | tar -xzC /usr/local/bin --transform 's#^.+/##x'
 
 WORKDIR /go/src/github.com/containous/traefik
-
-COPY glide.yaml glide.yaml
-COPY glide.lock glide.lock
-RUN glide install -v
-
-COPY integration/glide.yaml integration/glide.yaml
-COPY integration/glide.lock integration/glide.lock
-RUN cd integration && glide install
-
 COPY . /go/src/github.com/containous/traefik