voltctl - log configuration for multiple stacks doesn't work

voltctl log set/list command only works for the default VOLTHA stack. If there are multiple stacks available, the only way to set the log config for other stacks is to change the env variable KV_STORE_DATAPATH_PREFIX.

In default, KV_STORE_DATAPATH_PREFIX points to "service/voltha/voltha1_voltha1" path. 

When the "voltctl use stack" command is issued, the correct KV_STORE_DATAPATH_PREFIX should be set by the voltctl.

The ultimate solution would be to have a VOLTHA API to set component log levels. See VOL-4797.