Adds a CLI for entrypoint

[JoshuaGabriel]

move server startup behind a cobra command tree. Helps deploy ceph-api as a single binary for testing purposes and also later to add commands to hit certain endpoints e.g status/health

$ go build -o ceph-api ./cmd/ceph-api
$ ./ceph-api serve -h
Start the ceph-api server

Usage:
  ceph-api serve [flags]

Flags:
  -h, --help   help for serve

Global Flags:
      --config string            set path to config directory
      --config-override string   set path to config override directory




./ceph-api -h
Ceph API server and client

Usage:
  ceph-api [flags]
  ceph-api [command]

Available Commands:
  completion  Generate the autocompletion script for the specified shell
  help        Help about any command
  serve       Start the ceph-api server
  version     Print version information

Flags:
      --config string            set path to config directory
      --config-override string   set path to config override directory
  -h, --help                     help for ceph-api

Use "ceph-api [command] --help" for more information about a command.