roachtest: add ability to set default virtual cluster

[renatolabs]

This commit adds a SetDefaultVirtualCluster function to the cluster interface. This function's primary use case is to simulate the behaviour of the server.controller.default_target_cluster cluster setting, but for separate-process tenants. When this function is called, every subsequent call to Conn will connect to the provided virtual cluster (instead of system). Similary, expansions provided by roachprod (such as {pgurl}) will also use the new default virtual cluster. This functionality will enable the mixedversion framework to continue to provide the existing API when testing separate-process tenant upgrades. It could also be useful if roachtest itself wanted to provide this kind of randomized deployment model in each run.

Epic: none

Release note: None