fix(artifact-test): assign scylla_mgmt_agent_repo for amazon2
fix(artifact-test): assign scylla_mgmt_agent_repo for amazon2

scylla_mgmt_repo is for installing scylla manager server.
scylla_mgmt_agent_repo is for installing scylla manager agent.

Currently the default scylla_mgmt_agent_repo is for Ubuntu 20.04,
this patch added special repo for amazon2.

Signed-off-by: Amos Kong <eb01d8f828a6c9a20be4534e72e049aaf41503df@scylladb.com>

scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests