[cluster-test] use jumphost in env var if defined
[cluster-test] use jumphost in env var if defined

Added an option in cluster test runner to use jumphost specified in env
var if there is one defined.  Fall back to default if not.

Closes: #2391
Approved by: andll

aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core