roachtest: Add support for multliple workload nodes

[csgourav]

This changes allows roachtests to specify multiple workload nodes in test. Before this, we could add only one workload node.

Epic: none Release note: None

[cockroach-teamcity]

This change is 

[csgourav]

TFTR! bors r+

[craig[bot]]

Build failed (retrying...):

• docker_image_amd64

[craig[bot]]

Build succeeded:

• local_roachtest_fips
• linux_amd64_build
• local_roachtest
• acceptance
• macos_amd64_build
• docker_image_amd64
• unit_tests
• windows_build
• examples_orms
• macos_arm64_build
• check_generated_code
• linux_amd64_fips_build
• linux_arm64_build
• lint