(maint) Make PDB container wait longer on Postgres

[Iristyle]

This is another approach to the PR against PDB at puppetlabs/puppetdb#2904

• PDB has a configurable wait time on Postgres when it is starting up, which
  defaults to 90 seconds

  This doesn't appear to be long enough in Azure based on failures like:

   wtfc.sh: waiting 90 seconds for nc -v -w 1 -z 'postgres' 5432
   .........................................................................................(/docker-entrypoint.d/20-wait-for-hosts.sh) Error: host postgres:5432 does not appear to be listening
   wtfc.sh: timeout occurred after waiting 90 seconds for nc -v -w 1 -z 'postgres' 5432 to return status: 0 (was status: 143)

[Iristyle]

Quite certain the problem here was not the wait time, but rather interesting DNS resolution problems, ultimately fixed in #61