[FIX] Current versions of rocketchat

[r0zbot]

This PR fixes the ansible role to work with current versions of rocketchat and supported OSs.

Other notable changes include:

• Changed GPG keyserver from ha.pool.sks-keyservers.net to keyserver.ubuntu.com, since it was deprecated and no longer works.

• Add support for Centos8

• Add support for Ubuntu 20.04

• Drop support for Ubuntu 14.04

• Updated node version to 12.22.1

• Updated mongodb version to 4.4

• Fixed syntax for compatibility with ansible 2.10+

• Other bugfixes

• Vagrant tests:

  • Added Ubuntu 20.04
  • Added Debian 10
  • Added Centos 8
  • Removed Fedora 27
  • Removed Ubuntu 14

[r0zbot]

This should close #113, #105, #104, #103

[florisvangeel]

Confirmes this works for getting RV 6.x up and running when using an external mongodb role
I used this pull request: https://github.com/graylog-labs/ansible-role-mongodb/tree/mongodb-5.0
Also not merged yet into its upstream.