Set dfs.client.failover.random.order to true as default

Currently, the default value of dfs.client.failover.random.order is
false. If it's true, clients access to NameNodes random order instead
of the configured order which is defined in hdfs-site.xml.

Setting dfs.client.failover.random.order=true is very important for
RBF if there are multiple routers. If it's false, all the clients
point to the same router because routers are always active.

And I think dfs.client.failover.random.order=true would be good manner
for normal HA(two-NameNodes) Cluster too. If it's false and the first
NameNode is standby, clients always access to standby NameNode at
first.

So I'd like to set dfs.client.failover.random.order to true as default
from 3.4. Does anyone have any concerns?