CLI: Show candidate devices

[joseivanlopez]

Problem

The information about the selected storage candidate devices is not shown by agama config show command.

Note: this is a Rust introductory card. The goal is to get familiar with the code and start writing some bits. The format of the storage settings (including the candidate devices) is going to be changed soon, see #656.

Solution

Loads the information about the candidate devices.

Bonus: print a new line for text and json formats.

$ agama -f json config show 
{"user":{"fullName":"","userName":"","password":"","autologin":false},"root":{"sshPublicKey":null},
"software":{"product":"Tumbleweed"},"storage":{"lvm":null,"encryptionPassword":null,
"devices":[{"name":"/dev/vdb"}]},"network":{"connections":[{"id":"enp1s0","method":"auto"},
{"id":"lo","method":"manual","addresses":["127.0.0.1/8"]}]}}
$

Testing

• Tested manually

[coveralls]

coverage: 72.339% (-0.06%) from 72.398% when pulling d35c294 on joseivanlopez:cli-storage-boot-device into f33d147 on openSUSE:master.

[imobachgs]

LGTM but, please, add the entry to the changes file.