Code review comment for lp:~therp-nl/openupgrade-server/7.0_lp1265463_use_password

@Holger

I changed the code to properly test for the existance of a configuration key before use.

I took over the unsetting of PGPASSWORD from the database back-up code of standard OpenERP. I think it is not wrong to clear a plaintext password from memory as soon as possible. Although ofcourse there already is a certain security risk in having the password in a plain text config file in the first place.