stop ticking the BT once N victims found

Top-down approach:

[x] find a way to dynamically specify the variable when starting the application per docker compose ...
- [x] change in .env file
- [x] specify for the run - research
[x] pass the value of the set env var to the roslaunch command in pkg's Dockerfile
[x] find a way do dynamically specify the variable N before launching the planner pkg
- [x] idea: specify arg tag and substitution
[x] get the variable passed to roslaunch from Dockerfile using ROS Parameter servers functions
[x] TEST break while loop in which ticking BT if blackboard variable representing number of victim rescued reaches N
[x] the variable should be set at the end of successful tick of rescue subtree - add new leaf node incrementing it
- idea: follow py_trees.blackboard.SetBlackboardVariable implementation and use py_trees.utilities.static_variables ?

srothh / ppmrob