rebase: vendor dependencies for Vault API

Uses github.com/libopenstorage/secrets to communicate with Vault. This
removes the need for maintaining our own limited Vault APIs.

By adding the new dependency, several other packages got updated in the
process. Unused indirect dependencies have been removed from go.mod.

Signed-off-by: Niels de Vos <ndevos@redhat.com>

vendor/github.com/grpc-ecosystem/go-grpc-middleware/.travis.yml

@@ -1,12 +1,18 @@
 sudo: false
 language: go
 go:
-  - 1.11.x
-  - 1.12.x
-
+  - 1.8.x
 env:
-  global:
-    - GO111MODULE=on
+  - DEP_VERSION="0.3.2"
+
+before_install:
+  # Download the binary to bin folder in $GOPATH
+  - curl -L -s https://github.com/golang/dep/releases/download/v${DEP_VERSION}/dep-linux-amd64 -o $GOPATH/bin/dep
+  # Make the binary executable
+  - chmod +x $GOPATH/bin/dep
+
+install:
+  - dep ensure
 
 script:
  - make test