[MT-68] Add ControlMaster ssh options in mila init

[lebrice]

• Adds the ControlMaster, ControlPath and ControlPersist options to the mila ssh configuration entry as well as the one used to connect to compute nodes.
• Updates the entries in-place if they already exist.

These options make it possible to reuse an existing ssh connection via what's called "SSH multiplexing". I'm not an expert, but basically, it seems to create a file in a predetermined path (set via "ControlPath") for each "real" ssh connection. Then, if you try to make a new SSH connection to the same host and port, that initial connection is reused instead of making a new one.

TL;DR: This makes it a LOT quicker to connect to the cluster or to compute nodes: after the first connection to the login node or compute node is done (still takes a while), the following connections are near-instant.

ALSO:

• Removes extra newlines between SSH configuration entries

  • This currently has a potentially annoying side-effect of collapsing existing empty block of > 2 empty lines in the user's ~/.ssh/config file into just a single empty line.

• Improves the name and format of regression files used in unit tests. They are now readable Markdown files in this format:

  Running the `mila init` command with this initial content:
  
  (the initial content)
  
  and these use inputs: (the user inputs)
  leads to the following ssh config contents:
  
  (the resulting content of ~/.ssh/config)
  

(See https://github.com/lebrice/milatools/blob/ControlMaster-mila-init-MT-68/tests/cli/test_init_command/test_with_existing_entries_has_mila_compute_entry_has_mila_cpu_entry_no_mila_entry_.md for an example)