zuul: further split merger and server

zuul::wikimedia::instance now invokes both zuul::merger and zuul::server
and parameters have been redistributed between them.

The 'zuul' class is now way simpler and just install zuul from source
and realize common things such as package dependencies or creating
directories.

Change-Id: Ia0d7a80f3b738e18f5f79b7e6ed32b82a7259e30