Cleaned up pilot_agent handling

* Multicore agent was hardcoded in launcher. Agent selection has been moved into
the resource configuration files under the key 'pilot_agent'. Additionaly, I
have added a 'pilot_agent_options' key so that it possible to pass specific
flags to the agent (e.g., -l LOCAL to the singe-core agent.)

* radical-agent-default.py has been renamed to radical-agent-singlecore.py for
symmetry with radical-agent-multicore.py

* radical-agent-multicore.py ("MPI agent") is now default for *ALL* HPC clusters.
radical-agent-singlecore.py is only used for 'localhost'.

Depending on the stability of the MPI agent, making the 'MPI agent' default on
all HPC resource might cause some disruption. However, this will also help us to
find bugs quickly :-)