[claudiubelu]

Test import provider's deploy_replica_disks() allocated a new, empty loop device on every execution, ignoring the volumes_info from the previous one. This dropped the disks' replica_state, turning every transfer execution into a full sync, and leaked the previously allocated loop devices.

Disks allocated by a previous execution are now reused, like the real import providers do.

The replicator only loads its state file at startup, and it is already running when replicate_disks() is called: deploy_replica_source_resources() starts it without any state. It was only restarted with the replica state
on reused pool minions, so incremental executions on fresh source minions were always full syncs.

The replicator is now always restarted with the replica state for incremental executions.

Disk integrity verification is now enabled for both executions of test_incremental_replica_transfer. The execution if the source and destination checksums differ, so the transferred content is verified
for any provider pair, not only with the core test import provider.

The test also asserts that the incremental execution wrote less data than the full one, which was not checked before: every execution was a full sync with the core test providers, and the test still passed.

Test providers can now implement exec_test_vm, which runs a shell command inside the test instance, and mutate_test_instance_data is built on top of it. The same hook can then be used by other tests that need to act on the guest.

Also adds ssh_utils, so providers whose test instances are reachable over SSH only have to supply the instance's address.

The core test export provider's instances were raw loop devices, so the only way to change their data was a hard-coded write at an offset. Instances are now formatted with a tiny ext4 filesystem holding only the host's static busybox, and exec_test_vm chroots into it. This keeps the transfer size small, compared to writing a full OS image.

Document the new busybox requirement in the integration tests README, and install it in the integration tests workflow.