Fix broken sstableloader tests

Neither the exit status of the sstableloader subprocess
or the stress subprocess to read and validate the data were
checking exit statuses, so failures went unnoticed.  In particular,
the tests failed to catch CASSANDRA-6205.

This patch does the following:
 * Fixes incompressible_data_in_compressed_table_test, which seemed to
   be totally broken
 * Instead of using stress to insert and validate data (ccm doesn't
   check exit statuses for stress), a normal cursor is used.
 * The exit status of the sstableloader subprocess is checked
 * Cleanup of unused imports, bad whitespace, and other pep8 violations