Allow constant zipped parameter in campaign incremental

[msimonin]

Currently if one specifies:

test_case_1:
  nbr_clients: [1, 2, 3]
  nbr_servers: [1, 1, 1]

We expect the incremental campaign to start (1, 1) then (1, 0) then (1, 0) clients and servers.
But I observed that the above results in starting (1, 1) then (1, 1) then (1, 1) clients and servers.

We want to support constant parameters in the zipped parameters.

---

The function generate_current_values (

ombt-orchestrator/orchestrator/campaign.py

Line 82 in 9204476

def get_current_values(params, current, key):

)
and the line :

ombt-orchestrator/orchestrator/campaign.py

Lines 92 to 93 in 9204476

	previous_clients, current_clients = get_current_values(parameters, current_parameters, 'nbr_clients')
	previous_servers, current_servers = get_current_values(parameters, current_parameters, 'nbr_servers')

causes the above behaviour.

Indeed parameters are treated separately leading the second line to always have previous_servers=0 and current_servers=0. This is due to the plist.index to return the first index of the occurence in a list.