CLI waiter-ssh should allow all users to specify container to SSH into

We should add support to the waiter ssh command to allow user to specify the container:

  -c, --container='': Container name. If omitted, use the waiter-app container by default

This is already available but limited only to admins:

waiter/cli/waiter/subcommands/ssh.py

Lines 180 to 183 in 8a0bd98

    
           if is_admin_enabled(): 
        
               parser.add_argument('--container-name', '-c', 
        
                                   help='specify the container name you want to ssh into. Defaults to "waiter-app". Has no ' 
        
                                        'effect if instance is not k8s pod.')

The default container continues to be waiter-app.

	if is_admin_enabled():
	parser.add_argument('--container-name', '-c',
	help='specify the container name you want to ssh into. Defaults to "waiter-app". Has no '
	'effect if instance is not k8s pod.')