Add local file embedding for SSH keys, systemd units and dropins

[travier]

With https://github.com/coreos/fcct/pull/99, we can use the following FCC to embed files from a directory in a config:

storage:
  files:
    contents:
      local: foo.conf

I would like to use a similar mechanism to embed:

• SSH Keys
• systemd units
• systemd dropins

[jlebon]

Not sure if this came up before, though some YAML parsers allow support for custom tags like !include: myfile which would solve this at a more generic level. (Edit: looking at https://github.com/go-yaml/yaml/pull/433, it seems like this is supported.)