Update a number of short descriptions to be 100 characters or less to reflect the new Hub length restriction

@fredlf if you have time to glance over these and make sure I've done a decent job, that'd be appreciated (but if not, that's OK too, just let us know)

Here's the relevant character counts after this change:
```console
$ wc -L */README-short.txt | sort -n
  12 hello-world/README-short.txt
  19 busybox/README-short.txt
  24 nginx/README-short.txt
  26 mageia/README-short.txt
  26 ubuntu/README-short.txt
  29 centos/README-short.txt
  29 jenkins/README-short.txt
  29 registry/README-short.txt
  34 debian/README-short.txt
  34 fedora/README-short.txt
  59 cirros/README-short.txt
  71 opensuse/README-short.txt
  72 scratch/README-short.txt
  76 crux/README-short.txt
  80 golang/README-short.txt
  81 clojure/README-short.txt
  81 perl/README-short.txt
  81 rails/README-short.txt
  84 java/README-short.txt
  84 mongo/README-short.txt
  86 redis/README-short.txt
  89 python/README-short.txt
  92 gcc/README-short.txt
  92 node/README-short.txt
  94 neurodebian/README-short.txt
  94 ubuntu-upstart/README-short.txt
  95 mysql/README-short.txt
  96 buildpack-deps/README-short.txt
  96 hylang/README-short.txt
  96 jruby/README-short.txt
  97 docker-dev/README-short.txt
  98 ruby/README-short.txt
  99 hipache/README-short.txt
  99 postgres/README-short.txt
  99 wordpress/README-short.txt
 100 php/README-short.txt
 100 ubuntu-debootstrap/README-short.txt
```

I realize we lose quite a bit of information in some places with this change, but it's information that's duplicated in the long description too, so should be generally OK.
Shorten to: 
MySQL is a widely used, open-source relational database management system (RDBMS).