调整instance_account唯一索引

[nick2wang]

#2108 新增的mongo账号管理添加的db_name字段没有加入唯一索引，导致同一mongo实例不同数据库录入相同名称账号时报错失败

MySQLdb._exceptions.IntegrityError: (1062, "Duplicate entry '11-hzpTW5c0st1JwqCNx3BFAA==-' for key 'instance_account_instance_id_user_host_514c1ac6_uniq'")

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (4bca447) 75.24% compared to head (5ea83e9) 75.24%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2179   +/-   ##
=======================================
  Coverage   75.24%   75.24%           
=======================================
  Files         105      105           
  Lines       15182    15182           
=======================================
  Hits        11424    11424           
  Misses       3758     3758           

Impacted Files	Coverage Δ
sql/models.py	96.57% <100.00%> (ø)

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