add config checking

[WindzCUHK]

crazy user prevention

TDOO

• [x] for role token exp, "" => 0, invalid duration => crash
• [x] add checking on other configs

[codecov-io]

Codecov Report

Merging #22 into master will decrease coverage by 0.3%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
- Coverage   96.66%   96.36%   -0.31%     
==========================================
  Files          10       10              
  Lines         569      577       +8     
==========================================
+ Hits          550      556       +6     
- Misses         10       12       +2     
  Partials        9        9

Impacted Files	Coverage Δ
service/role.go	97.35% <100%> (+0.11%)	:arrow_up:
usecase/tenantd.go	91.3% <0%> (-4.35%)	:arrow_down:

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[kevindiu]

LGTM