Add custom Git commit message template

Writing a proper commit message tends to be quite a complicated task.

Git allows adding a commit message template to every future commit
through its commit.template configuration variable. The commit message
template can be used to display best practices for writing commit
messages, at least until we are comfortable writing them on our own.
Update template for Git commit

The recommendations added to ~/.gitmessage are largely based on
OpenStack guidelines for writing good commit messages. For more
information see: https://wiki.openstack.org/wiki/GitCommitMessages
Merge feature-gitmessage into development

* feature-gitmessage:
  Update template for Git commit
  Add custom Git commit message template