Add bash completion for `plugin install --disable-content-trust`

Signed-off-by: Harald Albers <github@albersweb.de>
(cherry picked from commit 725a864a0496a6c10f56c30f8a45234230951343)
Signed-off-by: Victor Vieux <vieux@docker.com>

contrib/completion/bash/docker

@@ -3320,7 +3320,7 @@ _docker_plugin_install() {
 
 	case "$cur" in
 		-*)
-			COMPREPLY=( $( compgen -W "--alias --disable --grant-all-permissions --help" -- "$cur" ) )
+			COMPREPLY=( $( compgen -W "--alias --disable --disable-content-trust=false --grant-all-permissions --help" -- "$cur" ) )
 			;;
 	esac
 }