Fix #2174 - Fix RSpec rake targets

The target generation was wrong, as it created some
strange target for isolated test files.
It now creates two types of targets:

• directory targets, like:

rake integration/network/server
or
rake unit/network

Those run all tests in a specific directory

• spec targets like:

rake unit:indirector:catalog:yaml
or
rake unit:parser:ast:definition

Signed-off-by: Brice Figureau <brice-puppet@daysofwonder.com>