(TK-50) Require user to specify default server

Require user to specify a default server when using a multiserver
configuration. Update docs and example to reflect this change.
(TK-50) Change default server configuration

Change the configuration of a default server to be a boolean
contained in the map for one server in a multiserver set-up.
Make the default server specification optional.
(TK-50) Remove atom

Remove atom being used in the get-default-server-from-config
function and replace it with a combination of first and filter.
(TK-50) Address PR feedback

Address the feedback on the PR for this ticket.
Merge pull request #52 from fpringvaldsen/TK-50-specify-default

(TK-50) Require user to specify default server