checksum support in builder.

Using *sha* instead of *sha1* because createrepo documentation says:
```
-s --checksum
              Choose the checksum type used in repomd.xml and for packages in the metadata.  The default is now "sha256"  (if  python  has
              hashlib).  The  older  default  was  "sha",  which is actually "sha1", however explicitly using "sha1" doesn't work on older
              (3.0.x) versions of yum, you need to specify "sha".
```