placeholders for mysql/postgre installers

Since I am a going to mess up with Jenkins jobs to implement
Mysql/PostgreSQL based jobs, I want to make sure we bail out properly
whenever the job is being used.

Will eventually refactor mw-install* script somehow, maybe using a
shared script with symlinks pointing to it, the $0 would give the
database to install.

Change-Id: I7eed97253003ca1aa14bc36fcbbba4bb99a4b6df