Namenodes not been filtered by fs.defaultFS

[alanhe]

Steps to reproduce:
Add another cluster configuration in the testdata file: https://github.com/colinmarc/hdfs/blob/master/hadoopconf/testdata/conf/hdfs-site.xml#L37
to make it:

<configuration>
    <property>
        <name>dfs.nameservices</name>
        <value>tests</value>
    </property>
    <property>
       <name>dfs.ha.automatic-failover.enabled</name>
       <value>true</value>
    </property>
    <property>
        <name>dfs.ha.namenodes.mycluster</name>
        <value>nn1,nn2</value>
    </property>
    <property>
        <name>dfs.namenode.rpc-address.mycluster.nn1</name>
        <value>namenode1:8020</value>
    </property>
    <property>
        <name>dfs.namenode.rpc-address.mycluster.nn2</name>
        <value>namenode2:8020</value>
    </property>
    <property>
        <name>dfs.ha.namenodes.yourcluster</name>
        <value>nn1,nn2</value>
    </property>
    <property>
        <name>dfs.namenode.rpc-address.yourcluster.nn1</name>
        <value>namenode3:8020</value>
    </property>
    <property>
        <name>dfs.namenode.rpc-address.yourcluster.nn2</name>
        <value>namenode4:8020</value>
    </property>
</configuration>

Add a test case in hadoopconf_test.go

func TestDefaultFSAddress(t *testing.T) {
	conf, err := Load("testdata/conf")
	assert.NoError(t, err)

	addresses := conf.Namenodes()
	assert.Equal(t, 2, len(addresses))
}

Because the fs.defaultFS is hdfs://mycluster in testdata/conf/core-site.xml, I want to see 2 addresses been returned, but there are 4.