Fix ConnectionFactory auto-configuration with XA

Previously, the regular jms connection factory was always overriding
the xaConnectionFactory when using ActiveMQ. While
ActiveMQAutoConfiguration is namely shielded with a condition on a
missing ConnectionFactory bean, said configuration class also imports the
XA and regular configuration classes that both can create the connection
factory.

This commit adds a ConditionalOnMissingBean in the second class that is
imported in case the XA configuration has already defined what it needs.

Fixes gh-1727

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jms/activemq/ActiveMQConnectionFactoryConfiguration.java

@@ -20,6 +20,8 @@ import javax.jms.ConnectionFactory;
 
 import org.apache.activemq.ActiveMQConnectionFactory;
 import org.apache.activemq.pool.PooledConnectionFactory;
+
+import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 
@@ -32,6 +34,7 @@ import org.springframework.context.annotation.Configuration;
  * @since 1.1.0
  */
 @Configuration
+@ConditionalOnMissingBean(ConnectionFactory.class)
 class ActiveMQConnectionFactoryConfiguration {
 
 	@Bean