Added multi-threading support for API

Usage
--------
make test-foreman-api-threaded

Before
------
Ran 759 tests in 1608.000s

FAILED (SKIP=99, errors=25, failures=10)
make: *** [test-foreman-api] Error 1

After
-----
Ran 742 tests in 287.268s

FAILED (SKIP=99, errors=12, failures=8)
make: *** [test-foreman-api] Error 1

Most additional errors are 500 Server Errors.
Because IDs are issued using sequences, we should not encounter
erroneous results, due to ID collisions.