Manual configuration settings for kernel_shmmax and kernel_shmall are ignored on postgresql deployments

When performance_tuning is set to either "auto" or "manual", any juju config settings for kernel_shmmax and kernel_shmall are ignored. When writing the sysctl file /etc/sysctl.d/50-postgresql.conf, any manual settings for kernel_shmmax or kernel_shmall should override guesses during performance_tuning= "auto". If performance_tuning is set to "manual" those shmmax and shmall values should be written to this file as well.

 Although juju configuration parameter kernel_shmall is specified in terms of bytes, it should be written in terms of number of pages when creating /etc/sysctl.d/50-postgresql.conf.