Change MS DEFAULT_MIN_REWARD to Raiden's default

[karlb]

We don't have a communication channel to establish a good reward during
runtime, so using the same fixed value is best in the meantime.
This also prevents MS operators from accidentally not demanding a
reward, which is a bad idea since they have to pay transaction fees.

[codecov]

Codecov Report

Merging #773 into master will decrease coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #773      +/-   ##
==========================================
- Coverage   90.54%   90.46%   -0.08%     
==========================================
  Files          35       35              
  Lines        2590     2591       +1     
  Branches      329      329              
==========================================
- Hits         2345     2344       -1     
- Misses        178      180       +2     
  Partials       67       67              

Impacted Files	Coverage Δ
src/monitoring_service/cli.py	83.78% <100.00%> (ø)
src/monitoring_service/constants.py	100.00% <100.00%> (ø)
src/pathfinding_service/api.py	94.86% <0.00%> (-0.80%)	⬇️

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[Dominik1999]

lgtm thanks!