Fix recurent problem of port not free when starting mongo daemon.

For instance:
https://test.savoirfairelinux.com/job/Shinken-Upstream-Commit-Short-Tests/697/testReport/junit/%3Cnose/suite/ContextSuite_context_TestConfigSmall__setup/

simply create a new socket with random port selection, and use that port for the mongo daemon.
import 'unittest' from shinken_test.

and explicitely define unittest2 in our test/requirements.txt.
Merge pull request #7 from shinken-monitoring/fix_recurent_port_not_free

Fix recurent port not free