puppetize researchdb password file

put the password to the research database
into a file in the filesystem of stat1003

let the new 'researchers' admin group read
the file, for:

RT #7105 - sharing credentials for internal
           SQL slaves

this way:

- we have a clean 'researchers' group we can just
  add people to who should know this password

- if the password changes it just needs to be updated
  once in the private puppet repo and this file will change
  automatically

- Dario doesn't have to manually handle it at all

I added the researchers group in Ic0b31593bec0881 but I
had to revert that in I973c2fca6c73c because of a dependency
on the wikidev group on stat1003, we'll need to work
with Andrew to get that dependency removed in analytics puppet classes

Change-Id: I30b59c7b705a127aff9394e3facefcc8c2f3481b