Fix migration error in CRON type of CI. (#2606)

* Wait response from API `/server/about` before creating superuser in cron tests
Co-authored-by: NKruchinin <dmitryx.kruchinin@intel.com>

.travis.yml

@@ -23,6 +23,7 @@ env:
     DJANGO_SU_EMAIL="admin@localhost.company"
     DJANGO_SU_PASSWORD="12qwaszx"
     NODE_VERSION="12"
+    API_ABOUT_PAGE="localhost:8080/api/v1/server/about"
 
 before_install:
   - nvm install ${NODE_VERSION}
@@ -34,7 +35,7 @@ script:
   - if [[ $TRAVIS_EVENT_TYPE == "cron" && $TRAVIS_BRANCH == "develop" ]];
     then
     docker-compose -f docker-compose.yml -f ./tests/docker-compose.email.yml up -d --build;
-    docker exec -it cvat bash -ic 'python3 ~/manage.py migrate';
+    bash -c 'while [[ "$(curl -s -o /dev/null -w ''%{http_code}'' ${API_ABOUT_PAGE})" != "401" ]]; do sleep 5; done';
     docker exec -it cvat bash -ic "echo \"from django.contrib.auth.models import User; User.objects.create_superuser('${DJANGO_SU_NAME}', '${DJANGO_SU_EMAIL}', '${DJANGO_SU_PASSWORD}')\" | python3 ~/manage.py shell";
     cd ./tests && npm install && npm run cypress:run:firefox; exit $?;
     fi;