Add missing `can_push` parameter to POST /v3/deploy_keys

The v3 API documentation claims that `can_push` is supported when
adding a deploy_key.

https://gitlab.com/gitlab-org/gitlab-ce/blob/8-16-stable/doc/api/deploy_keys.md#add-deploy-key

Here we make good on this promise by adding `can_push` as an optional
parameter so grape can route it.

This was copied from the v4 API.

lib/api/v3/deploy_keys.rb

@@ -41,6 +41,7 @@ module API
           params do
             requires :key, type: String, desc: 'The new deploy key'
             requires :title, type: String, desc: 'The name of the deploy key'
+            optional :can_push, type: Boolean, desc: "Can deploy key push to the project's repository"
           end
           post ":id/#{path}" do
             params[:key].strip!