Normalize database connection parameters

[cecton]

Since Odoo 8.0 is a bit... complicated about the connection variables, I suggest we could "normalize" the connection parameter to set the --db_* argument AND PG* env variables to make sure all the situations are covered.

Current Priorities

• --db_host, then PGHOST, ... (then localhost or /run/postgresql)
• --db_port, then PGPORT, ... (5432)
• --db_user, then LOGNAME, then die
• --db_password, then PGPASSWORD

During a backup on Odoo 8.0, it's a bit different...

• --db_host, then PGHOST, ... (then localhost or /run/postgresql)
• --db_port, then PGPORT, ... (5432)
• --db_user, then LOGNAME, then die
• PGPASSWORD, then --db_password, then none

[sle-odoo]

We've finally been bitten by this kind of things (see a3d207f). I'm closing this as we now only use the --db* arguments in the Docker file but we have opened a task to clarify and fix these issues upstream.

Regards