Enable environment variable in action name

[ryutoyasugi]

Issue

#1145

Test

$ cat manifest.yaml
packages:
  default:
    actions:
      ${ACTION_NAME}:
        function: index.js
        runtime: nodejs
    triggers:
      ${TRIGGER_NAME}:
        feed: /whisk.system/alarms/alarm
    rules:
      ${RULE_NAME}:
        action: ${ACTION_NAME}
        trigger: ${TRIGGER_NAME}

$ export ACTION_NAME=hello
$ export TRIGGER_NAME=alarm
$ export RULE_NAME=hello-alarm

$ go build -o wskdeploy
$ ./wskdeploy --preview
Packages:
Name: default
    bindings:
    annotation:

  * action: hello
    bindings:
    annotation:


Triggers:
* trigger: alarm
    bindings:
    annotation:
        - feed : /whisk.system/alarms/alarm

Rules
* rule: hello-alarm
    annotation:
    - trigger: alarm
    - action: hello