[zookeeper][service_discovery] rootPath support customize，not just "/services"

[xiaomiusa87]

What would you like to be added:
Use zookeeper as registryCenter，expected customize 'rootPath'，not just "/services"

Why is this needed:
We use 'rootPath' to manage multiple clusters， want to call Java through dubbo-go

[FoghostCn]

why not use different application name or group name to manage multiple clusters

[xiaomiusa87]

why not use different application name or group name to manage multiple clusters

application name or group name is business attributes，has already been used.
we are middleware developer ，plan multiple clusters by business type. may use multi zk or one zk，so need use different
rootPath