Switching lettuce.config to lettuce.CORE

Now given_a_setup_test_environment() in mpas_tasks.py has logic to
figure out which core is being run and then open the core-specific
config file.
I have deleted lettuce.config and added lettuce.ocean lettuce.landice.
Adding post-scenario hook to return to basedir

This is so if a scenario has an unexpected error that leaves PWD in some
strange place, the next scenario still has a chance to run correctly.
Add flags=... option in lettuce.CORE config file

These flags will be added to the 'make' command used to compile the
model.  They should be space-delimited and unquoted, like:
flags = DEBUG=true ALBANY=true

I have also rewritten a lot of given_a_setup_test_environment() so there
is:
1. A loop over the list ('trusted', 'testing') so that the test
environment setup is identical for each (and easier to maintain).
2. Changed subprocess.call to subprocess.check_call so that if the
command being called fails, an error occurs.