fix easegress-server keep running on invalid yaml

[suchen-sci]

This PR fix issue #431. If user provides invalid yaml, then easegress-server will report error and return.

ps: Here invalid yaml file means it provides wrong type for option. For example, option.cluster.initial-cluster has type map[string]string; if we provide an []string instead it will cause problem.

[codecov-commenter]

Codecov Report

Merging #432 (9f99afb) into main (8ae49a6) will decrease coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #432      +/-   ##
==========================================
- Coverage   80.47%   80.38%   -0.10%     
==========================================
  Files          70       70              
  Lines        8093     8094       +1     
==========================================
- Hits         6513     6506       -7     
- Misses       1222     1228       +6     
- Partials      358      360       +2     

Impacted Files	Coverage Δ
pkg/object/mqttproxy/broker.go	75.05% <100.00%> (+0.05%)	⬆️
pkg/cluster/cluster.go	49.65% <0.00%> (-1.03%)	⬇️
pkg/object/mqttproxy/client.go	77.67% <0.00%> (-0.94%)	⬇️

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[suchen-sci]

in mqtt, we store session asynchronous. And this cause some mqtt test about session failed occasional (we check before session stored). this pr will fix them too.