provider/joyent/config.go: panic bug #1300846

provider/joyent/config.go: panic bug #1300846

When key-file wasn't set, it defaults to either the MANTA_KEY_FILE
environment variable or "~/.ssh/id_rsa". We then use the contents of
key-file to populate the "private-key" setting (which is the actual
contents of the file).

I wonder if we should actually have it named "private-key-path" to
match how other config variables are done.

Anyway, this just moves when we populate private-key. So instead of
populating it in "prepare" we do it in "validate" right after we've
ensured we have valid "key-file" contents.

I have not tested this live, because I don't have a Joyent account, but
the local tests all pass.

https://code.launchpad.net/~jameinel/juju-core/1.18-nil-key-file-1300846/+merge/213805

(do not edit description out of merge proposal)

[jameinel, 2014-04-02 10:37:46]

Please take a look.

[jameinel, 2014-04-02 10:51:48]

Please take a look.

[wallyworld, 2014-04-02 11:51:31]

I tested live without a key-file attr set and looks ok. So LGTM.
I agree that we should rename the key-file parameter before landing (including
the MANTA_KEY_FILE env var)