Check for invalid --agent.tags

[esnible]

If --agent.tags (or other tags) are malformed, display the invalid value as part of the panic.

No related Github issue. Found this while testing the operator. Improves an error message that will appear in the pod log if a misconfiguration keeps the Jaeger Query deployment from starting.

Old message:

panic: runtime error: index out of range [1] with length 1

New Message

panic: invalid Jaeger tag pair "dummy", expected key=value

Reproduce by building locally and running ~/go/src/github.com/jaegertracing/jaeger/cmd/query/query-darwin-amd64 --agent.tags=dummy

[codecov]

Codecov Report

Merging #3246 (e4eb68b) into master (27fe841) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3246      +/-   ##
==========================================
+ Coverage   95.98%   96.00%   +0.02%     
==========================================
  Files         242      242              
  Lines       14813    14815       +2     
==========================================
+ Hits        14218    14223       +5     
+ Misses        515      513       -2     
+ Partials       80       79       -1     

Impacted Files	Coverage Δ
cmd/flags/flags.go	48.21% <100.00%> (+1.91%)	⬆️
pkg/config/tlscfg/cert_watcher.go	92.63% <0.00%> (-2.11%)	⬇️
plugin/storage/integration/integration.go	79.28% <0.00%> (+0.39%)	⬆️
cmd/query/app/static_handler.go	97.60% <0.00%> (+1.79%)	⬆️
cmd/collector/app/server/zipkin.go	70.73% <0.00%> (+2.43%)	⬆️

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