Maya: Use of multiple deadline servers

[antirotor]

Feature

This feature allows to use multiple deadline webservice instance in Maya and allow user to choose from. When preset exists in pype-config/presets/plugins/maya/create.json:

{
  "CreateRender": {
    "deadline_servers": {
      "default": "DEADLINE_REST_URL",
      "aws": "http://foo:2135"
    }
  }
}

Then when render instance is created in Maya, it will add Deadline Servers combo box to its attributes:

It uses key as alias and values as its url. DEADLINE_REST_URL will be replaced by the one set with the same named environment variable. When user select different server, pools are queried from it.
Collector will then "bake" this value to deadlineUrl in published instance and rest of the plugins will use that.

Note: when this preset is not set, it will work as usual with DEADLINE_REST_URL env var set.

Closes pypeclub/client#60
🎫 Freshdesk: #642