[cluster-test] ct: use master image by default
[cluster-test] ct: use master image by default

Most of people don't want to build cluster test, only use it, so for them best option is to use latest master image

Added `-L` flag to use latest image built on host

Closes: #2233
Approved by: ankushagarwal

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