Fix Btrfs subvolume snapshot

[a1ro]

What does this PR do?

Hi,

I tried to snapshot a Btrfs subvolume with Salt, but the btrfs.subvolume_snapshot module didn't work on my machine.
I ran this module from both a salt state on master and a salt-call on minion. Either way, I got an error message.

Looking at the code for this module, I believe the source argument was missing while building the cmd list.
By appending source to the cmd list, a snapshot is now created and the command returns successfully.

Previous Behavior

Error message on the minion:

[ERROR   ] Command 'btrfs' failed with return code: 1
[ERROR   ] stderr: btrfs subvolume snapshot: exactly 2 arguments expected, 1 given
[ERROR   ] retcode: 1

Seems like the final command was: btrfs subvolume snapshot [-r] <dest>.

New Behavior

Minion output:

local:
    True

The correct command should be: btrfs subvolume snapshot [-r] <source> <dest>.
This PR adds the missing <source> argument.

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

Commits signed with GPG?

No

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[Ch3LL]

This will require a changelog and test coverage.

A good place to start is here: tests/pytests/unit/states/test_btrfs.py

[welcome]

Congratulations on your first PR being merged! 🎉