Set new API for setting last security updates timestamp

Unit tests are not working yet...
Fix unit tests that are not working yet
Pass schemas dir in fake as env var

ZENTYAL_MODULES_SCHEMAS is used to set the directory in EBox::Config::TestStub::fake
Unit tests are now passed
Merge pull request #253 from Zentyal/ejhernandez/set-sec-updates-last-time

Ejhernandez/set sec updates last time