Config/Engine: Add data directory to config.json (#549)

* add data directory to config.json

* fix quality check issues

* adjust data directory only when explicitly set

* unexport ValidateSettings
* process flags earlier so they can also be used when loading config
* fix test depends on flags

* rename config.DataDir to DataDirectory

* also don't omit in JSON if empty

* datadir flag induces dry run

* log warning
* enable parallel for sub tests
* leave data dir empty in example config

* remove parallel for loadConfigWithSettings

* create a new config object instead of using a shared one

* remove a test that potentially reads user file

* rename test methods to MixedCaps

* clean up test dir after engine tests

* use global config variable

config_example.json

@@ -1,5 +1,6 @@
 {
  "name": "Skynet",
+ "dataDirectory": "",
  "encryptConfig": 0,
  "globalHTTPTimeout": 15000000000,
  "database": {