postgres_set does not allow reuse of $variable

[KangOl]

see [1]

Why this restriction?
This forbid users to factorise the configuration and put postgres_set directive inside a file that will be included in multiple server

[1] https://github.com/FRiCKLE/ngx_postgres/blob/master/src/ngx_postgres_module.c#L1097