Enable passing cloud-init config via stdin

[gerboland]

No description provided.

[townsend2010]

Cool! A couple of comments already and I'll test the functionality a little later.

[townsend2010]

Good stuff here. However, if a user puts in incorrect YAML, yaml-cpp will throw and then the output passed back to the client is kind of vague.

For example, I did this:

$ multipass launch --cloud-init -
foo
man
chu<Ctrl-D>
launch failed: operator[] call on a scalar

Not a very helpful message as it's not clear where that really came from (although in this extreme case, it is:)) But I imagine a slight error in the YAML may produce a similar error message and then it's not so clear.

My thinking is that we put the YAML::Load in launch() and perhaps prepare_user_data() in a try/catch block so we can catch this and translate it into at least a better error message indicating it's coming from the cloud-init YAML parsing.

[townsend2010]

@gerboland,

After thinking about it, the yaml-cpp exceptions I mentioned should be handled as a separate issue as that can impact a passed in file as well and is out of scope for this PR. As such, this is good and I'm approving.