Updating `--shared-preload-libraries` is highly sensitive to formatting

[davissp14]

It's currently super easy for someone to blow away the repmgr reference when updating shared-preload-libraries. When a user needs to specify multiple values it's also important that they wrap the value inside a single quote.

For example:

This formatting will lead to breakage:

fly pg config update -a app-db --shared-preload-libraries "repmgr,pg_stat_statements"

This is the correct formatting:

fly pg config update -a app-db --shared-preload-libraries "'repmgr,pg_stat_statements'"

There's a couple improvements we need to make here:

1. repmgr should never be removed. If the user attempts to remove it, we should just include the reference by default.
2. We should work the value has the proper formatting before sending it to the VM.