[JCLOUDS-279] Remove spurious executable bit
Add test for correct and incorrect contentMD5

Tested against atmos (skipped), aws-s3, azureblob, cloudfiles-us, and
filesystem (skipped).
Add Atmos-specific MD5 checksum header

Tested against Atmos Online.
JCLOUDS-277: Made the description in org.jclouds.compute.domain.Image nullable
Skip unimplemented test with transient blobstore
JCLOUDS-269: Report ram as MB rather than GB for SoftLayer