nixos/mesos: Parameterize mesos slave attributes

Added attributes to nixos/tests/mesos.nix to verify that mesos-slave
attributes work. If the generated attributes are invalid, the daemon
should fail to start.

Change-Id: I5511245add30aba658b1af22cd7355b0bbf5d15c
Merge pull request #5378 from benley/mesos

mesos-slave: add config option for slave attributes