SGF-289 - Enumeration restrictions (xsd:enumeration) should be avoided in the XML schema.

Removed the 'subscriptionPolicyType' XSD simple-type definition and subsequently, all Subscription policy enumeration restrictions on SDG Region elements in the SDG XML namespace (e.g. gfe:partitioned-region) that declare and define a Subscription policy.  Renamed SubscriptionType to InterestPolicyType along with the associated test suite class.  Renamed the SubscriptionTypeConverter class to InterestPolicyConverter along with the associated test suite class.  Created the RegionSubscriptionAttributesNamespaceTest class to test the configuration of Subscription on GemFire Regions in the SDG XML namespace using various settings and property placeholders.

src/test/resources/org/springframework/data/gemfire/config/RegionSubscriptionAttributesNamespaceTest-context.xml

@@ -0,0 +1,42 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<beans xmlns="http://www.springframework.org/schema/beans"
+	   xmlns:context="http://www.springframework.org/schema/context"
+	   xmlns:gfe="http://www.springframework.org/schema/gemfire"
+	   xmlns:util="http://www.springframework.org/schema/util"
+	   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+	   xsi:schemaLocation="
+	   	http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
+	   	http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd
+		http://www.springframework.org/schema/gemfire http://www.springframework.org/schema/gemfire/spring-gemfire.xsd
+	   	http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util.xsd
+">
+
+	<util:properties id="subscriptionProperties">
+		<prop key="subscription.interest.policy">ALL</prop>
+	</util:properties>
+
+	<context:property-placeholder properties-ref="subscriptionProperties"/>
+
+	<util:properties id="gemfireProperties">
+		<prop key="name">RegionSubscriptionAttributesNamespaceTest</prop>
+		<prop key="mcast-port">0</prop>
+		<prop key="log-level">config</prop>
+	</util:properties>
+
+	<gfe:cache properties-ref="gemfireProperties"/>
+
+	<gfe:replicated-region id="NoSubscriptionRegion" persistent="false"/>
+
+	<gfe:replicated-region id="AllSubscriptionRegion" persistent="false">
+		<gfe:subscription type="${subscription.interest.policy}"/>
+	</gfe:replicated-region>
+
+	<gfe:partitioned-region id="CacheContentSubscriptionRegion">
+		<gfe:subscription type="Cache_ConTENt"/>
+	</gfe:partitioned-region>
+
+	<gfe:partitioned-region id="DefaultSubscriptionRegion">
+		<gfe:subscription/>
+	</gfe:partitioned-region>
+
+</beans>