Replace wikimedia-raid-utils by up-to-date megacli

wikimedia-raid-utils contains an ancient version of megacli that doesn't
support several of our systems (at least rdb100x & external shelves on
ms100x). Instead of keeping this up-to-date, add an external apt
repository to reprepro's update that has all kinds of tools packaged &
install megacli out of there.

Since this is one of the few binary things that we use, introduce a new
non-free component in our apt and place the binaries there, as to make
it clear these comew without source

A cursory look didn't reveal other controllers being used across the
infrastructure except megacli, so be conservative and only include that.
If check-raid complains, we can always add more packages from that repo,
as it seems to have packaged tools for more than wikimedia-raid-utils
ever covered.

Change-Id: I3279af7bbcee8906c0cbaa73a089876651de4da8