MIR: mesa-vulkan-drivers FFE: install by default

MIR mesa-vulkan-drivers:
- is from mesa, which is already in main
- maintained just as mesa is, by ubuntu-x-swat

MIR vulkan-loader:
https://bugs.launchpad.net/ubuntu/+source/vulkan-loader/+bug/1742711

FFE:
- new packages in the default install (mesa-vulkan-drivers, libvulkan1)
  -> no chance of regressing anything

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A Vulkan related bug was filed at https://bugs.launchpad.net/ubuntu/+source/vulkan/+bug/1742711, but in a related note, having mesa-vulkan-drivers moved from Universe to Main and installed by default in Ubuntu 18.04 is crucial. Vulkan has gotten mature and since the OpenGL drivers are installed, it will very complimentary.

Having this available will aid game developers and users who load up a Vulkan application and get errors. They may not know to enable Universe in their Software Manager and to manually install mesa-vulkan-drivers for their video card. They may just load up that Vulkan application, find out it doesn't work, and then give up on it. We cannot have that for future adoption of the API.

Plus, GTK and Qt are adopting Vulkan and applications like Gnome MPV are implementing experimental support for Vulkan. For Ubuntu being a leader for the desktop and a base for GNU/Linux gaming, shouldn't it be painless for Ubuntu 18.04? Especially since it is a long term release for 5 years.