Ensure artifacts in project dist area include required md5 file

From the 0.98.19RC0 thread:

busbey

[1]: ASF policy requires that each file hosted in the project dist
space have a file with just the MD5 sum in a file named after the
original with ".md5" as a suffix. (Having an additional file with all
the checksums is a good practice, IMO.) I brought this up in our last
round of RCs as well. I don't want to hold up this vote, but I plan to
start voting -1 on future RCs that don't include md5 files.

relevant policy:
http://www.apache.org/dev/release-distribution.html#sigs-and-sums

apurtell

Our release documentation (https://hbase.apache.org/book.html#releasing)
says we should generate sums like so:

for i in *.tar.gz; do echo $i; gpg --print-mds $i > $i.mds ; done

The make_rc.sh script also encodes the same. Let's fix.