Change default value for dfs.ha.nn.not-become-active-in-safemode to false

[soenkeliebau]

Description

Change default value for dfs.ha.nn.not-become-active-in-safemode from true to false to be in line with the Hadoop default value.

This caused issues during a cold restart of an HDFS cluster for a user, due to both namenodes refusing to become active, hence creating a deadlock.

The relevant code in HDFS is shown below, and since this will never be skipped if notBecomeActiveInSafemode is true and the namenode is in safemode it can potentially delay startup a long time or even indefinitely.

    if (notBecomeActiveInSafemode && isInSafeMode()) {
      throw new HealthCheckFailedException("The NameNode is configured to " +
          "report UNHEALTHY to ZKFC in Safemode.");
    }

fixes #264

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