Add support for ES6 style async boot scripts

[lehni]

Description

This PR allows boot scripts to be defined in the ES6 async fashion, without the need for passing callbacks:

module.exports = async function(app) {
   ...
};

Related issues

• None

Checklist

• New tests added or existing tests modified to cover all changes
• Code conforms with the style
  guide

[slnode]

Can one of the admins verify this patch?

[slnode]

Can one of the admins verify this patch?

[slnode]

Can one of the admins verify this patch?

[slnode]

Can one of the admins verify this patch? To accept patch and trigger a build add comment ".ok\W+to\W+test."

[bajtos]

@raymondfeng @davidcheung please review. You may want to consider my comments in #252 too.

[bajtos]

@slnode ok to test

[bajtos]

Hi @lehni, could you please forward-port the changes from #252 that you made as part of the review process?

[lehni]

@bajtos I am back from my holiday, will look into these pending things now!

[bajtos]

@lehni sounds good, welcome back! :)

[lehni]

@bajtos this PR now contains the changes of both #252 and #260 ported over to the master branch.

[bajtos]

LGTM. Let's give some time for @raymondfeng to review too.