Add a command to list operation's predecessors

[rpignolet]

This use case is for debug. For example I want to run manually an operation (a.k.a. Ansible playbook). In order to run this operation I must run all DAG operation predecessors.

With the tdp deploy --targets ... command we can run a deployment to a list of targets so we only need direct operation's predecessors, not all, only direct ones. To get them we can use the pred property of networkx.

Here an example snippet to print a tdp deploy command with all ranger_admin_config direct operation's predecessors:

dag = Dag(...)
tdp_deploy_command = "tdp deploy --targets "
pred_operations = dag.graph.pred["ranger_admin_config"]
print(tdp_deploy_command + ','.join(pred_operations))

Outputs:

tdp deploy --targets tdp-cluster_config,hadoop_client_config,ranger_kerberos_install,ranger_ssl-tls_install

After running this tdp deploy command we can run manually, with ansible-playbook command, the ranger_admin_config playbook.