Bug fix for run once mode (#65)

* Change condition for run once mode from KSM_REFRESH_FREQUENCY_MS=-1 to KSM_REFRESH_FREQUENCY_MS=0

* Allow KSM_REFRESH_FREQUENCY_MS to be set to 0 or -1 for run once mode

* Enable run once mode if KSM_REFRESH_FREQUENCY_MS has negative or zero value

* Update documentation

Author: andriyfedorov

README.md

@@ -105,7 +105,7 @@ The [default configurations](src/main/resources/application.conf) can be overwri
 
 - `KSM_READONLY=false`: enables KSM to synchronize from an External ACL source. The default value is `true`, which prevents KSM from altering ACLs in Zookeeper
 - `KSM_EXTRACT=true`: enable extract mode (get all the ACLs from Kafka formatted as a CSV)
-- `KSM_REFRESH_FREQUENCY_MS=10000`: how often to check for changes in ACLs in Kafka and in the Source. 10000 ms by default. If it's set to `-1` then KMS executes ACL synchronization just once and exits
+- `KSM_REFRESH_FREQUENCY_MS=10000`: how often to check for changes in ACLs in Kafka and in the Source. 10000 ms by default. If it's set to `0` or negative value, for example `-1`, then KMS executes ACL synchronization just once and exits
 - `AUTHORIZER_CLASS`: authorizer class for ACL operations. Default is `SimpleAclAuthorizer`, configured with
   - `AUTHORIZER_ZOOKEEPER_CONNECT`: zookeeper connection string
   - `AUTHORIZER_ZOOKEEPER_SET_ACL=true` (default `false`): set to true if you want your ACLs in Zookeeper to be secure (you probably do want them to be secure) - when in doubt set as the same as your Kafka brokers.

src/main/scala/com/github/simplesteph/ksm/KafkaSecurityManager.scala

@@ -53,8 +53,8 @@ object KafkaSecurityManager extends App {
     })
 
     try {
-      //if appConfig.KSM.refreshFrequencyMs is equal to -1 the aclSyngronizer is run just once.
-      if(appConfig.KSM.refreshFrequencyMs == -1){
+      //if appConfig.KSM.refreshFrequencyMs is equal or less than 0 the aclSyngronizer is run just once.
+      if(appConfig.KSM.refreshFrequencyMs <= 0){
         log.info("Single run mode: ACL will be synchornized once.")
         aclSynchronizer.run()
       } else {