runservice: add step when conditions

[sgotti]

No description provided.

[huapox]

Should already be implemented? ref #73

Cur runs/tasks already works

• tasks

local task_gitsrc() = {
    name: 'git-src',
    runtime: {
      arch: 'amd64',
      containers: [{image: 'alpine/git'}],
    },
    steps: steps_gitsrc(),
    depends: [],
    when: {
      // branch: '#.*#',
      branch: '#master#',
      ref: '#refs/pull/\\d+/head#',
    },

• runs

{
  runs: [
    {
      name: 'agola build/test',
      tasks: [ ..
      ],
      when: {
        branch: '#.*#',
        #ref: '#refs/pull/\\d+/head#',
      },
    },
  ],
}