use mysql compatible configuration and allow duplicate keys in mysqlConf field

[usernameisnull]

related issue: #931
change spec.mysqlConf field: map[string]map[string]intstr.IntOrString -> string, compatible with original mysql ini configurations

• allow boolean configuration, only key
• allow duplicate field

like below:

mysqlConf: |-
    performance-schema # only key
    binlog-format = ROW
    binlog_do_db = db4
    binlog_do_db = db5

in the running mysql instance:

the conf in the running instance:

• I've made sure the CHANGELOG.md will remain up-to-date after this PR is merged.

[usernameisnull]

PTAL @cndoit18 @calind