fix/test Random failures of test.aria.core.environment.Customizations
fix/test Random failures of test.aria.core.environment.Customizations

This commit fixes random failures of test.aria.core.environment.Customizations,
by waiting for the load of the first customization descriptor to be
finished before loading another one.

closes #1599

fbasso/ariatemplates,divdavem/ariatemplates,simonarbuckle/ariatemplates,benouat/ariatemplates,vcarle/ariatemplates,ariatemplates/ariatemplates,simonarbuckle/ariatemplates,benouat/ariatemplates,divdavem/ariatemplates,vcarle/ariatemplates,ymeine/ariatemplates,ymeine/ariatemplates,vcarle/ariatemplates,ymeine/ariatemplates,fbasso/ariatemplates,ariatemplates/ariatemplates,ariatemplates/ariatemplates,divdavem/ariatemplates,fbasso/ariatemplates,simonarbuckle/ariatemplates,benouat/ariatemplates