Setup root FS for multibucket object storage

[MorrisJobke]

Uses first bucket of a multi bucket setup for the root FS

How to test

Install https://github.com/jubos/fake-s3 and start it:

sudo gem install fakes3
mkdir /tmp/s3
fakes3 -r /tmp/s3 -p 4567

Configure multibucket:

'objectstore_multibucket' => [
	'class' => 'OC\\Files\\ObjectStore\\S3',
	'arguments' => [
		'bucket' => 'abc',
		'num_buckets' => 64,
		'key' => '123',
		'secret' => 'abc',
		'hostname' => 'localhost',
		'port' => '4567',
		'use_ssl' => false,
		'use_path_style' => 'true',
	],
],

Check out that the data dir only holds the log file and no appdata_* folders.

[mention-bot]

@MorrisJobke, thanks for your PR! By analyzing the history of the files in this pull request, we identified @icewind1991, @LukasReschke and @Kondou-ger to be potential reviewers.

[MorrisJobke]

This is basically a copy of initObjectStoreRootFS() only the 5 lines above this comment are added.

[MorrisJobke]

I also added the fix for #3978 (comment) as well here:

 Use distributed cache for SCSS dependencies as well
    
    * followup to #3978 because with the proper setup of
      appdata we now are using the same data on all nodes

[rullzer]

Makes sense!
Tested with fakes3 and works!