Change how PostgreSQL role sets shared_buffers

By default, if 'postgresql_sysctl_shm_multiplier' is set, PostgreSQL
role modifies kernel SHMMAX value according to total system memory and
configures PostgreSQL shared_buffers according to total system memory
divied by number of clusters configured.

If 'postgresql_sysctl_shm_multiplier' is set to False, PostgreSQL role
will now configure 'shared_buffers' setting in each cluster according to
currently set SHMMAX value, substracting additional memory required for
'max_connections'.