Do not allow duplicate properties in cluster entities

When specifying properties for a cluster, this is currently allowed,

<properties>
        <property name="test" value="value1"/>
        <property name="test" value="value2"/>
</properties>

The propeties are stored as an array of org.apache.falcon.entity.v0.cluster.Property, and cluster.getProperty("test") will return either "value1" or "value2" but not both. If falcon does not support multiple values for same property key, parsing such an entity should throw an error.