[feat] Allow SSH configuration via CLI

[fubarhouse]

Pygmy should be able to write to a configuration file to configure SSH keys.

We should be able to utilize the configuration overrides system for this.

[fubarhouse]

So the way I would see this working is that we would use pygmy to build a configuration out and write it to a file.

For example, pygmy config set key value should generate something like the following:

pygmy/docs/customisation/introduction.md

Lines 105 to 107 in cb85053

	keys:
	- path: /home/user1/.ssh/id_rsa
	- path: /home/user2/.ssh/id_rsa

Ideally we should also have an unset command as well, but all references to the default value should be omitted if configuration is provided - an example of the hard-coding can be found here:

pygmy/cmd/up.go

Lines 78 to 79 in cb85053

	homedir, _ := homedir.Dir()
	keypath := fmt.Sprintf("%v%v.ssh%vid_rsa", homedir, string(os.PathSeparator), string(os.PathSeparator))

A lot of the heavy work setting defaults can be found in the setup file - example below:

pygmy/external/docker/setup/setup.go

Lines 148 to 150 in cb85053

	if c.Services["amazeeio-haproxy"].HostConfig.PortBindings == nil {
	c.Services["amazeeio-haproxy"] = GetService(haproxy.NewDefaultPorts(), c.Services["amazeeio-haproxy"])
	}

The difficult part to work out would be how to write out this setting, or remove it from existing configuration - without affecting the rest of the configuration. You can wipe your config and see what this looks like using pygmy export to see this in action but the output config can't be read back in.

@jackwrfuller please check it out and get back to me with any questions or comments, as you have asked to get involved a little.