some links are broken after renaming dubbo-ops to dubbo-admin,
though not all of the links are broken, they are updated as well
for consistency

* replace local constants with global constants

* remove unnecessary declarations

* create adapter for util method

* remove unused imports

* remove unnecessary code. issue #307

* unit test for ConfigCenter

* add authorization for zookeeper group

* change registry name

* change properties's key name

* retry

Fix string comparison
Fix possible index out of bound