Configurable production database name and credentials

[bemoody]

Allow configuring the production database name (and username and hostname) by bringing these old settings modules in line with the new style config variables used by physionet.settings.settings.

(All these variables should already be set correctly on all our servers, but of course we should check before deploying.)

[bemoody]

(And I think it should be "safe" to merge this because I think the git hooks should squawk if the variables are missing, but I'm testing that right now.)

[tompollard]

thanks!