Fixes#1533 and #1534
Memsetting `turn_params.default_users_db` before reading conf file, not
after.
Because auth is read in first iteration so secret was wiped out.
# test plan
Add new test script that uses config file to setup turnserver instead of
cli arguments and confirm it works (fails without the change)
Install dependencies and build with prometheus support during CI builds
Ubuntu 16.04 has issues supporting `MHD_USE_EPOLL_INTERNAL_THREAD` so
disabling it (using SELECT)
Should not impact anything as the only reason we use Ubuntu 16.04 is to
validate build against openssl-1.0.2
Fixes #998
