Moved sample network creation out of unittest base class constructor.

This sample networks only needed in:

 nova.tests.api.ec2.test_cloud
 nova.tests.integrated.test_api_samples
 nova.tests.network.test_manager
 nova.tests.test_libvirt
 nova.tests.test_xenapi

Moreover, base class constructor is not good place for sample
data creation. Database should be empty before runing tests.

Fixes: bug 1181502

Change-Id: I8dcaa5b36a1da0c0928c9445c556d788d463844e
Merge "Moved sample network creation out of unittest base class constructor."