cel_pgsql: Add support for setting schema

Add feature to set optional schema parameter on configuration file via
'schema' setting.

Fix query to get columns from table while considering schema. If in
the database there exists two tables with same name in distinct schemas
it will return an error when inserting record.

ASTERISK-24967 #close

Change-Id: I691fd2cbc277fcba10e615f5884f8de5d8152f2c