Support for key_file fstab option

[vaygr]

This adds basic support for key_file fstab option, which can be used for mounting with specific credentials (like for cross-project buckets and VMs).

It's quite useful, because mount.gcsfuse doesn't respect GOOGLE_APPLICATION_CREDENTIALS env var, but only gcsfuse does. But even if it did, it would be quite hacky to make it work under systems like Puppet, for example.

The option simplifies this by a lot:

my-bucket /mount/point gcsfuse rw,noauto,user,key_file=/path/to/key.json

it re-uses already existing --key-file argument for gcsfuse.

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[vaygr]

I signed it!

[googlebot]

CLAs look good, thanks!

[jacobsa]

Please run gofmt.

[vaygr]

I did. Double-checked that now.

[jacobsa]

My apologies, I had misjudged what gofmt would do here. Thanks!

[jacobsa]

Thanks for this! I've left a few comments.

[jacobsa]

Actually this looks good as-is; will accept and touch up if I feel it needs it later. Thanks!