[issue4] updated terrain scripts to support file based logs.

RunningProcess now manages the separate log file, and tweak the configuration
so log messages will be dumpted to that file, not to stderr.
new wait_for_log_str() will search the log file directory.
[issue4] update feature files so they'll refer to logs, not stderr.
[issue4] embed placeholder keyword for log output options in config files.

also added README to explain the expectation.
Merge pull request #5 from bundy-dns/issue4

fix to Issue4: lettuce failure due to corrupted logs