add user parameter for db connection

[nichtsfrei]

What:

under some circumstances the db username is not the same as the local
user. To support that usecase the parameter db-user is added.

Why:

To enable socket connection into an container with trust instead of peer configuration.

How:

Start a docker container with with a installed postgresql, mount -v /tmp/test:/var/run/postgresql:z , change peer to trust in it via:

 RUN sed -i 's/peer/trust/' /etc/postgresql/11/main/pg_hba.conf

start a migration with a postgres known username and db:

/usr/local/sbin/gvmd --db-host=/tmp/test/ -d gvmd --db-user gvm -m

Checklist:

• Tests
• CHANGELOG Entry

[mattmundell]

Looks OK.

Could you add a change log entry, please?