add driveAlias

[micbar]

Description

Implements the base features for space aliases as a space property in the decomposedFS

Detailed description

owncloud/ocis#3283 (comment)

[micbar]

I discovered a possible problem in

reva/pkg/storage/utils/decomposedfs/spaces.go

Line 223 in 40f7f00

if link[0:10] != "....spaces" || link[46:51] != "nodes" {

We cannot assume that the length is as we expect it to be. I found that during unit testing.
../../spaces/4c/510ada-c86b-4815-8820-42cdf82c3d51/nodes/4c/51/0a/da/-c86b-4815-8820-42cdf82c3d51 could be
../../spaces/us/erid/nodes/4c/51/0a/da/-c86b-4815-8820-42cdf82c3d51

1. Create a user with a userid which is not UUIDv4
2. Log in, we will create a personal space with the userid as spaceID
3. the string will be shorter as we expect it

[kobergj]

Just a small question

[micbar]

@kobergj I added configurable template strings.

[kobergj]

Looks still good to me 👍

[micbar]

general approach needs to be discussed

@dragotin FYI