Make update module and artifact generator which installs other update modules

Acceptance criteria:

• Artifact generator must produce an artifact containing one or more update modules
• Artifact must contain information about which version the module is meant for
  • This can be hardcoded (v3) in the generator for now, but must be present in the artifact for future compatibility
• Installer must install the modules into the directory specified by the version in the artifact:
  • /usr/share/mender/modules/<version>
• Module must support rollback
  • To avoid breaking updates
  • Should be easy
• Module must be installed by default using:
  • meta-mender
  • mender-convert
• Must be tested (most likely in client acceptance tests)