Added `rm -rf target/` to the "elasticsearch:build" Rake tasks

... since `mvn clean` doesn't seem to do the job sometimes... :/
[EXT] Increased the default logging level for the testing cluster to `DEBUG`

The tests were failing constantly because `DiskThresholdDecider` prevented shards being allocated when there was less than 10% of disk space available and it wasn't printed in the log on older Elasticsearch versions.
[EXT] Disabled `allocation.disk.threshold_enabled` in the testing cluster to prevent tests failing due to low disk space

The tests were failing constantly because `DiskThresholdDecider` prevented shards being allocated when there was less than 10% of disk space available and it wasn't printed in the log on older Elasticsearch versions.

The log lines:

    [2014-11-10 14:38:43,463][TRACE][cluster.routing.allocation.allocator] [node-1] No Node found to assign shard [[myindex][3], node[null], [P], s[UNASSIGNED]]
    [2014-11-10 14:38:43,463][DEBUG][cluster.routing.allocation.decider] [node-1] Node [aOmGFlU7RfmnKuRRU86u3g] has 1.2534550422124373% free disk
    [2014-11-10 14:38:43,463][DEBUG][cluster.routing.allocation.decider] [node-1] Less than the required 10.0 free bytes threshold (1.2534550422124373 bytes free) on node aOmGFlU7RfmnKuRRU86u3g, preventing allocation even though primary has never been allocated
    [2014-11-10 14:38:43,463][TRACE][cluster.routing.allocation.decider] [node-1] Can not allocate [[myindex][4], node[null], [P], s[UNASSIGNED]] on node [aOmGFlU7RfmnKuRRU86u3g] due to [DiskThresholdDecider]
    [2014-11-10 14:38:43,463][DEBUG][cluster.routing.allocation.decider] [node-1] Node [-jaQy2_ISruHQhTyy6_o7A] has 1.2534550422124373% free disk
    [2014-11-10 14:38:43,463][DEBUG][cluster.routing.allocation.decider] [node-1] Less than the required 10.0 free bytes threshold (1.2534550422124373 bytes free) on node -jaQy2_ISruHQhTyy6_o7A, preventing allocation even though primary has never been allocated