Should reset Balancer.Parameters.DEFALUT.policy in TestBalancer

Now in TestBalancer.java, more than one test case will invoke the private method runBalancer, in which it will use Balancer.Parameters.Default, while the policy is never reset thus its totalUsedSpace and totalCapacity will increase continuously.

We can reveal this issue by simply change

testUnevenDistribution(conf,
        new long[] {50*CAPACITY/100, 10*CAPACITY/100},
        new long[]{CAPACITY, CAPACITY},
        new String[] {RACK0, RACK1});

to

testUnevenDistribution(conf,
        new long[] {70*CAPACITY/100, 40*CAPACITY/100},
        new long[]{CAPACITY, CAPACITY},
        new String[] {RACK0, RACK1});

which in current implement, will cause none node under-replication thus cause the test case fail