checksums: use sha256 checksums everywhere

We'd like to use a consistent checksum scheme everywhere so that we can:

    a) incorporate archive checksums into our specs and have a
       consistent hashing algorithm across all specs.

    b) index mirrors with a consistent type of checksum, and not one that
       is dependent on how spack packages are written.

- [x] convert existing md5, sha224, sha512, sha1 checksums to sha256

var/spack/repos/builtin/packages/iozone/package.py

@@ -14,7 +14,7 @@ class Iozone(MakefilePackage):
     homepage = "http://www.iozone.org/"
     url      = "http://www.iozone.org/src/current/iozone3_465.tar"
 
-    version('3_465', 'c924e5e46fb1cf8145f420e8e57eb954')
+    version('3_465', sha256='2e3d72916e7d7340a7c505fc0c3d28553fcc5ff2daf41d811368e55bd4e6a293')
 
     # TODO: Add support for other architectures as necessary
     build_targets = ['linux-AMD64']