Custom validation in integration tests

[ipspace]

Sometimes we can't use the "standard" netlab.validate command to validate platform functionality (for example, graph generation).

The "device-module-test" script should use the defaults.netlab.integration_test dictionary to get commands for individual steps (or skip steps if there are no commands).

After the "netlab create" command (which has to be done anyway), the script should load the pickled snapshot, get the commands to execute, and do the rest of the test as instructed in lab topology.