cloud: aws: check md5 when pulling data
cloud: aws: check md5 when pulling data

Currently dvc downloads data from cloud even if we already
have the same file downloaded. This patch fixes it by checking
md5 before attempting to download.

Fixes #159

Signed-off-by: Ruslan Kuprieiev <f6538312ce47017d734b1357aabc970ebf896e32@gmail.com>

dataversioncontrol/dvc,dataversioncontrol/dvc,efiop/dvc,dmpetrov/dataversioncontrol,efiop/dvc,ophiry/dvc,ophiry/dvc,dmpetrov/dataversioncontrol