Bluetooth: Mesh: Remove _cli postfix from models shell files

This commits renames `health_cli.c` and `cfg_cli.c` under `mesh/shell`
folder to `health.c` and `cfg.c` accordingly. This is to create a
generic pattern for the future, when not only clients, but server models
will also expose some API through shell commands. This means that server
models' shell commands should be located in the same file as client
commands to avoid cluttering `mesh models` subcommand.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>