S3A: debug logging for http traffic to S3 stores

AWS SDK bundle.jar logging doesn't set up right.

SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.

Cloudstore commands have a -debug option to force set this through log4j APIs; this does work.

Proposed:

• add reflection-based ability to set/query log4j log levels (+tests, obviously)
• add a new log `org.apache.hadoop.fs.s3a.logging.sdk`
• if set to DEBUG, DefaultS3ClientFactory will enable logging on the aws internal/shaded classes

this allows log4j.properties to turn on logging; reflection ensures all is well on other log back-ends and when unshaded aws sdk jars are in use