Broker failed to start when specifying a list of bookie groups

[murong00]

Describe the bug
When set bookkeeperClientIsolationGroups to a list of bookie groups (e.g. group1,group2), pulsar service failed to start:

18:33:22.906 [main] ERROR org.apache.pulsar.PulsarBrokerStarter - Failed to start pulsar service.
org.apache.pulsar.broker.PulsarServerException: java.lang.ClassCastException: java.util.ArrayList cannot be cast to java.lang.String
        at org.apache.pulsar.broker.PulsarService.start(PulsarService.java:526) ~[pulsar-broker-2.6.0-SNAPSHOT.jar:2.6.0-SNAPSHOT]
        at org.apache.pulsar.PulsarBrokerStarter$BrokerStarter.start(PulsarBrokerStarter.java:274) ~[pulsar-broker-2.6.0-SNAPSHOT.jar:2.6.0-SNAPSHOT]
        at org.apache.pulsar.PulsarBrokerStarter.main(PulsarBrokerStarter.java:339) [pulsar-broker-2.6.0-SNAPSHOT.jar:2.6.0-SNAPSHOT]

Expected behavior
Pulsar service should start successfully when specifying a list of bookie groups.

Describe the solution you'd like
Add type judgment when get String property.