enable profile_task callback plugin by default

Question: do we want to enable callback plugin by default?

prints the time it takes for each task as well as a summary at the end
the output is cluttered after this change and one can't change it much, one can increase the number of tasks shown at the end with environment variable PROFILE_TASKS_TASK_OUTPUT_LIMIT 1
seeing as many updates to ansible lately are about improving the performance this could be useful.

$ ansible-playbook compute.yml -t cuda -D -l gpu

PLAY [grab facts from install nodes] *******************************************
skipping: no hosts matched

PLAY [Compute Nodes] ***********************************************************

TASK [setup] *******************************************************************
Monday 29 August 2016  09:04:41 +0300 (0:00:01.100)       0:00:01.100 ********* 
ok: [gpu]

PLAY [GPU Nodes] ***************************************************************

TASK [ansible-role-cuda : add nvidia CUDA repo] ********************************
Monday 29 August 2016  09:04:43 +0300 (0:00:01.648)       0:00:02.748 ********* 
ok: [gpu]

..SNIP..

TASK [ansible-role-cuda : Initialize the GPUs if there is no /dev/nvidia0] *****
Monday 29 August 2016  09:04:53 +0300 (0:00:00.143)       0:00:12.326 ********* 
skipping: [gpu]

PLAY RECAP *********************************************************************
gpu                        : ok=9    changed=0    unreachable=0    failed=0   

Monday 29 August 2016  09:04:53 +0300 (0:00:00.081)       0:00:12.408 ********* 
=============================================================================== 
ansible-role-cuda : add nvidia CUDA repo -------------------------------- 1.85s
setup ------------------------------------------------------------------- 1.65s
ansible-role-cuda : template in cuda_init.sh used during boot ----------- 1.62s
ansible-role-cuda : template in cuda_init.service systemd script -------- 1.61s
ansible-role-cuda : install cuda software - this is slow - restart if cuda_restart_node_on_install is True --- 1.41s
ansible-role-cuda : enable the cuda_init systemd service ---------------- 1.09s
ansible-role-cuda : make sure cuda_init.sh script is absent from rc.local --- 0.97s
ansible-role-cuda : check if cuda_gpu_name0 ( /dev/nvidia0 ) exists ----- 0.89s
ansible-role-cuda : debug ----------------------------------------------- 0.14s
ansible-role-cuda : Initialize the GPUs if there is no /dev/nvidia0 ----- 0.08s

fgci-org / fgci-ansible

enable profile_task callback plugin by default #149