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/ip.go

@@ -0,0 +1,47 @@
+package opts
+
+import (
+	"fmt"
+	"net"
+)
+
+// IPOpt holds an IP. It is used to store values from CLI flags.
+type IPOpt struct {
+	*net.IP
+}
+
+// NewIPOpt creates a new IPOpt from a reference net.IP and a
+// string representation of an IP. If the string is not a valid
+// IP it will fallback to the specified reference.
+func NewIPOpt(ref *net.IP, defaultVal string) *IPOpt {
+	o := &IPOpt{
+		IP: ref,
+	}
+	o.Set(defaultVal)
+	return o
+}
+
+// Set sets an IPv4 or IPv6 address from a given string. If the given
+// string is not parseable as an IP address it returns an error.
+func (o *IPOpt) Set(val string) error {
+	ip := net.ParseIP(val)
+	if ip == nil {
+		return fmt.Errorf("%s is not an ip address", val)
+	}
+	*o.IP = ip
+	return nil
+}
+
+// String returns the IP address stored in the IPOpt. If stored IP is a
+// nil pointer, it returns an empty string.
+func (o *IPOpt) String() string {
+	if *o.IP == nil {
+		return ""
+	}
+	return o.IP.String()
+}
+
+// Type returns the type of the option
+func (o *IPOpt) Type() string {
+	return "ip"
+}