when two clients come for functions where check_default_vpc
is present then one goes to create default vpc and other fails.
it fails because vpc with is_default flag is already in DB
but children objects (like subnets) are not yet created.
For we will lock check_default_vpc function.
Another way is to rework _create_vpc. It can create VPC
without is_default flag. And at the end it can set the flag.
First thread will pass this. And second thread will fail at flag set
operation and will revert all created object. Then second thread
can check presence of default VPC again.
This way is better cause it can work across several controllers.
Change-Id: I5586fa234257b72721e328a9fa2375a56d1553c2
Andrey Pavlov