Add configurability for NameNode readiness criteria

[xkrogen]

When waiting for the Dynamometer infra application to be "ready" to use, the Client and ApplicationMaster wait for the following criteria to be met:

1. Number of live DataNodes is above a threshold
2. Number of missing blocks is under a threshold
3. Number of underreplicated blocks is under a threshold

These percentage-based thresholds are currently hard-coded, but it would be useful if they were configurable to give a user more control over how strict the readiness condition is.