clean up plugin model and add admitter support

[wlan0]

@rrati

Try this with this commit

# build plugin
go build -buildmode=plugin -o kube_installer.so plugin/examples/kube_installer.go

# build short
go build

# install plugin
./short plugin install kubernetes --path=./kube_installer.so

# list plugins
./short plugin ls
PLUGIN NAME              ADMITTER        INSTALLER
kubernetes                    true                     true

# activate plugins
./short plugin activate kubernetes

# list plugins
./short plugin ls
PLUGIN NAME              ADMITTER        INSTALLER
kubernetes                    true*                    true*

# install complex app using plugins and simple config
cat << EOF | ./short -k -
config_map:
  name: "test"
  data:
    name: "perceptor"
    image: "gcr.io/gke-verification/blackducksoftware/perceptor:latest"
    config_map: "perceptor-config"
    config_map_mount_path: "/etc/perceptor"
    port: "3001"
    cmd: "./perceptor"
EOF

# verify installation
kubectl get rc

[wlan0]

tagging @jayunit100 - didn't know handle off the top of my head