Add serverless application deploy command

Description

This merge request implement a serverless application deployment command.

This is a replacement for deploying applications with tm tool.

How does it work?

1. During the build stage an image gets pushed to GitLab Container Registry.
2. During the deploy stage the command below gets invoked.
3. It deploys a new service to a Knative cluster, using the image built before.
4. It is environment-aware, especially CI/CD-native, so you don't need to provide arguments like the image name unless you are deploying a custom image / want to override an application name.
5. One can override arguments using GITLAB_APP_ environment variables in a CI/CD job (variables: GITLAB_APP_NAME: something).

Example:

$ /usr/bin/gitlabktl application deploy

Closes #9 (closed)