Enable loading metadata from the hive metastore snapshot and cleanup build scripts.

This patch contains the following changes:
  - Add a metastore_snapshot_file parameter to build.sh
  - Enable skipping loading the metadata.
  - create-load-data.sh is refactored into functions.
  - A lot of scripts source impala-config, which creates a lot of log spew. This has now
    been muted.
  - Unecessary log spew from compute-table-stats has been muted.
  - build_thirdparty.sh determins its parallelism from the system, it was previously hard
    coded to 4
  - Only force load data of the particular dataset if a schema change is detected.

Change-Id: I909336451e5c1ca57d21f040eb94c0e831546837
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5540
Reviewed-by: Ishaan Joshi <ishaan@cloudera.com>
Tested-by: jenkins