Add prefer-const rule to eslint and fix the previous occurrences

[XVincentX]

Nothing fancy happening here.

[codecov]

Codecov Report

Merging #455 into master will not change coverage.
The diff coverage is 98.49%.

@@           Coverage Diff           @@
##           master     #455   +/-   ##
=======================================
  Coverage   90.89%   90.89%           
=======================================
  Files         226      226           
  Lines        9271     9271           
=======================================
  Hits         8427     8427           
  Misses        844      844

Impacted Files	Coverage Δ
test/routing/host-regex.test.js	96.29% <100%> (ø)	⬆️
lib/rest/routes/users.js	83.01% <100%> (ø)	⬆️
lib/admin/apps.js	100% <100%> (ø)	⬆️
test/routing/host-capture.all.test.js	95.45% <100%> (ø)	⬆️
lib/policies/key-auth/passport-apikey-strategy.js	86.36% <100%> (ø)	⬆️
test/services/tokens.test.js	77.96% <100%> (ø)	⬆️
lib/services/consumers/user.service.js	86.17% <100%> (ø)	⬆️
lib/gateway/pipelines.js	92.8% <100%> (ø)	⬆️
bin/environment.js	97.1% <100%> (ø)	⬆️
test/services/authorization-codes.test.js	96.96% <100%> (ø)	⬆️
... and 118 more

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