[#2064] hammer.py: ensure postgres entry in hba

Closes #2064 (closed)

It seems that the postgres user gets locked out of the database again.

https://jenkins.aws.isc.org/job/kea-dev/job/ut-extended/445

psql: FATAL:  password authentication failed for user "postgres"

I thought I tested this on all OSs, but apparently not. Some OSs are missing the postgres ident entry which I did not anticipate.