Add advanced secret management

**What**
- Add the ability to specify secrets as a csv
- Vendor the docker/cli/opts
- Update the guide for secrets to use the `faas-cli`

**Why**
- Allowing the csv specification of secrets gives users more control about how
those secrets are mounted into the container.  This is good for things like
key rotation and for developers that are building on top of OpenFaaS.

Signed-off-by: Lucas Roesler <lucas.roesler@gmail.com>

gateway/vendor/github.com/docker/cli/opts/opts_unix.go

@@ -0,0 +1,6 @@
+// +build !windows
+
+package opts
+
+// DefaultHTTPHost Default HTTP Host used if only port is provided to -H flag e.g. dockerd -H tcp://:8080
+const DefaultHTTPHost = "localhost"