Return a postgres connection.

def _get_conn():
    '''
    Return a postgres connection.
    '''
    try:
        conn = psycopg2.connect(
               host=__opts__['master_job_cache.postgres.host'],
               user=__opts__['master_job_cache.postgres.user'],
               password=__opts__['master_job_cache.postgres.passwd'],
               database=__opts__['master_job_cache.postgres.db'],
               port=__opts__['master_job_cache.postgres.port'])
    except psycopg2.OperationalError:
        log.error('Could not connect to SQL server: %s', sys.exc_info()[0])
        return None
    return conn