handle karaf bug in listFeatures

Karaf FeaturesService has a bug that (rarely) causes it to throw a ConcurrentModificationException thus preventing correct operation of the Config Subsystem.

2014-09-21 20:31:43,133 | ERROR | r - ConfigPusher | ConfigPushingRunnable | 142 - config-persister-feature-adapter - 0.2.5.SNAPSHOT | Exception while processing features {}
java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextEntry(HashMap.java:922)[:1.7.0_65]
at java.util.HashMap$KeyIterator.next(HashMap.java:956)[:1.7.0_65]
at java.util.AbstractCollection.toArray(AbstractCollection.java:195)[:1.7.0_65]
at org.apache.karaf.features.internal.FeaturesServiceImpl.listInstalledFeatures(FeaturesServiceImpl.java:754)[24:org.apache.karaf.features.core:3.0.1]
at Proxy7aa88c93_025d_476f_9e2e_252f1abd09a1.listInstalledFeatures(Unknown Source)[:]
at org.opendaylight.controller.configpusherfeature.internal.FeatureConfigPusher.isInstalled(FeatureConfigPusher.java:85)[142:config-persister-feature-adapter:0.2.5.SNAPSHOT]
at org.opendaylight.controller.configpusherfeature.internal.FeatureConfigPusher.pushConfig(FeatureConfigPusher.java:73)[142:config-persister-feature-adapter:0.2.5.SNAPSHOT]
at org.opendaylight.controller.configpusherfeature.internal.FeatureConfigPusher.pushConfigs(FeatureConfigPusher.java:63)[142:config-persister-feature-adapter:0.2.5.SNAPSHOT]
at org.opendaylight.controller.configpusherfeature.internal.ConfigPushingRunnable.processFeatureEvent(ConfigPushingRunnable.java:67)[142:config-persister-feature-adapter:0.2.5.SNAPSHOT]
at org.opendaylight.controller.configpusherfeature.internal.ConfigPushingRunnable.run(ConfigPushingRunnable.java:49)[142:config-persister-feature-adapter:0.2.5.SNAPSHOT]
at java.lang.Thread.run(Thread.java:745)[:1.7.0_65]