Move delete crashed runs var to Settings class

[LuukBlom]

Usage:

from windows terminal:

set DELETE_CRASHED_RUNS=False
python floodadapt_run.py

from linux terminal:

export DELETE_CRASHED_RUNS=True
python floodadapt_run.py

from a python file

from flood_adapt.config import Settings
# Put this somewhere in initialization
Settings(
  delete_crashed_runs=False
)
... # the rest of the script

Any time Settings() is called,

1. It first reads the arguments it was given.
2. Then it fills unset fields with environment variables.
3. If there are still unset fields, it will use the defaults defined in the Settings class.
4. Then it will run all validators, and update the environment with its current state.

[panosatha]

LGTM