graphviz: workaround dot crashing for empty graphs

My version of dot crashes for an empty graph with options. Generate
a totally empty graph definition in these cases.
app: do not accept registering a process server with the same name twice

It must be de-registered with #remove_process_server first to make
sure it releases the associated ressources.
app: move the loader setup into a separate method

This is mainly for clarity
app: style fix
actions: set the name of the return value in Profile#resolved_definition
app: define #loader on ProcessServerConfig for clarity
app: fix return value of Configuration#sim_process_server

It was returning the original process server object instead
of the simulation one.
app: cleanup logic in Configuration#use_deployment
app: explicitly specify the loader when calling Process.parse_run_options
app: do not actually connect to the local process server in simulation AND single mode

When we run in simulation mode AND in single mode, we do not need
the process server to perform our duties (all models are local).
Moreover, in this particular mode, model packs are usable.

So, just register a ModelOnlyServer in place of all process
servers.
app: fix test for single? in #connect_to_orocos_process_server

Since we are on Configuration, single? was simply returning false
instead of raising.
app: fix path to DEFAULT_PORT in connect_to_orocos_process_server
app: improve error message
app: fix the fake localhost client being registered in all cases
test: add a 'count' option to try_instanciate to benchmark the merge algorithm

The option duplicates the specified definitions 'count' times before
running the deployer.
test: fix multiple inclusion of modules

In Ruby 2.1, the modules *are* included multiple times which causes
setup and teardown to be called multiple times as well and in
general wreaking havoc
test: fix path to Roby::Test::Assertion
test: in it_should_be_self_contained, use definitions returned by #resolved_definition

Otherwise, they are not the "full" definitions (i.e. don't include
global DI)
test: minor additions
better error message in InvalidSelection

The error message now displays all the models that are provided
by the selected object.