Fix for AMQ-5306

Cleaned up logic around the use of comma separated destinations, sysTest.totalDests and producer.destComposite. The logic is as follows:

Given -Dproducer.destName=foo:
default (-DsysTest.totalDests=1)
* publishes to 1 destination - foo

-DsysTest.totalDests=2
* publishes to 2 destinations - foo.0, foo.1

Given -Dproducer.destName=foo,cheese:
default (-Dproducer.destComposite=false -DsysTest.totalDests=1)
* publishes to 2 destinations - foo and cheese

(-Dproducer.destComposite=false) -DsysTest.totalDests=2
* publishes to 4 destinations - foo.0, foo.1 and cheese.0, cheese.1

-Dproducer.destComposite=true (-DsysTest.totalDests=1)
* publishes to 1 composite destination - "foo,cheese"

-Dproducer.destComposite=true -DsysTest.totalDests=2
* publishes to 1 composite destination - "foo.0,foo.1,cheese.0,cheese.1"