Support arbitrary action value from parameter in query string

[filippopisano]

Support arbitrary values for action in the Build Create API handler

Current implementation

Currently, when we create a build via REST API there is no straightforward way to target a specific pipeline if more than one custom event is present

---
kind: pipeline
type: docker
name: myaction1

steps:
- name: test
  image: node:9.8.0
  commands:
  - "echo MyAction1"

trigger:
  events:
    - custom
---
kind: pipeline
type: docker
name: myaction2

steps:
- name: test
  image: node:9.8.0
  commands:
  - "echo MyAction2"

trigger:
  events:
    - custom

In the example above both pipelines will be executed when the Build Create endpoint is called.

New implementation

---
kind: pipeline
type: docker
name: myaction1

steps:
- name: test
  image: node:9.8.0
  commands:
  - "echo MyAction1"

trigger:
  events:
    - custom
  # New parameter passed via Query String
  action:
    - myaction1

---

kind: pipeline
type: docker
name: myaction2

steps:
- name: test
  image: node:9.8.0
  commands:
  - "echo MyAction2"

trigger:
  events:
    - custom

  # New parameter passed via Query String
  action:
    - myaction2

Adding action in the trigger section will allow us to target only specific pipelines with a simple paramenter in the REST API:
curl -X POST -H "Authorization: Bearer <token>" https://hostname.com/api/repos/filippopisano/drone-test-repo/builds?action=myaction1

and

curl -X POST -H "Authorization: Bearer <token>" https://hostname.com/api/repos/filippopisano/drone-test-repo/builds?action=myaction2

[CLAassistant]

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[tphoney]

thanks for the PR, merged !