Allow overriding Munin script URLs with an environment variable

[hummeltech]

In order to at least partially support monitoring deployments not listening on localhost and/or using HTTPS.

E.g.:

[mod_tile_fresh]
env.MOD_TILE_URL = {url of /mod_tile endpoint}

[mod_tile_latency]
env.MOD_TILE_URL = {url of /mod_tile endpoint}

[mod_tile_response]
env.MOD_TILE_URL = {url of /mod_tile endpoint}

[mod_tile_zoom]
env.MOD_TILE_URL = {url of /mod_tile endpoint}

See here for documentation.

Resolves #418

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.40%. Comparing base (743cf65) to head (40bf4e7).
Report is 1 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #420      +/-   ##
==========================================
+ Coverage   79.98%   80.40%   +0.41%     
==========================================
  Files          24       24              
  Lines        5926     5925       -1     
==========================================
+ Hits         4740     4764      +24     
+ Misses       1186     1161      -25     

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