Fix flakiness of virtual SONiC tests

[mwindower]

By increasing sleep before restarting bgp on leaf switches.
Otherwise default route might be missing.

[Gerrit91]

Would be waiting for the default route a good waiting condition? As sleeps might depend on the performance of the host system?

[mwindower]

@Gerrit91 thanks for the hint.
This would look like this:

• If the default route is not visible on the machines we initiate a restart of bgp.service on the leaves
• After that we wait x seconds for the restart and bgp convergance.
• This would also dependent on the performance of the host.

As of now we stay with the simple sleep.