QA: Allow logging at different log levels

Problem to solve

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22084 implements basic debug logging. But when enabled it's very verbose and there's only one level - all or nothing.

Proposal

Change QA_DEBUG to QA_LOG_LEVEL and allow the value to set Logger's log level. If no value is set, logging is disabled. E.g., based on what @godfat proposed:

@logger.level = ::Logger.const_get((ENV['QA_LOG_LEVEL'] || 'DEBUG').upcase)

...If nothing set, no log. If it's set, it should be either DEBUG, INFO, and so on.

For consistency we may want to use Runtime::Env instead of accessing ENV directly