Split the dashboard and webserver recipes

In order to be able to use alternative webservers, split the parts
relating to setting up the dashboard itself and setting up the webserver
into two new recipes.

Also introduce a new configuration variable, defaulting to `apache2`,
which will be used in the `server` recipe to select the type of
webserver being installed.

Change-Id: I70dcb820239547b0059ad15d19d5e1689ddff3d3
blueprint: dashboard-split-horizon-apache