Expose backend type via REST API

This change will expose the user backend via the REST API which is a pre-requisite for https://github.com/owncloud/core/issues/12620.

For example:
````json
[{
    "name": "9707A09E-CA9A-4ABE-A66A-3F632F16C409",
    "displayname": "Document Conversion User Account",
    "groups": [],
    "subadmin": [],
    "quota": "default",
    "storageLocation": "\/Users\/lreschke\/Programming\/core\/data\/9707A09E-CA9A-4ABE-A66A-3F632F16C409",
    "lastLogin": 0,
    "backend": "OCA\\user_ldap\\USER_LDAP"
}, {
    "name": "ED86733E-745C-4E4D-90CB-278A9737DB3C",
    "displayname": "Hacker",
    "groups": [],
    "subadmin": [],
    "quota": "default",
    "storageLocation": "\/Users\/lreschke\/Programming\/core\/data\/ED86733E-745C-4E4D-90CB-278A9737DB3C",
    "lastLogin": 0,
    "backend": "OCA\\user_ldap\\USER_LDAP"
}, {
    "name": "71CDF45B-E125-450D-983C-D9192F36EC88",
    "displayname": "admin",
    "groups": [],
    "subadmin": [],
    "quota": "default",
    "storageLocation": "\/Users\/lreschke\/Programming\/core\/data\/71CDF45B-E125-450D-983C-D9192F36EC88",
    "lastLogin": 0,
    "backend": "OCA\\user_ldap\\USER_LDAP"
}, {
    "name": "admin",
    "displayname": "admin",
    "groups": ["admin"],
    "subadmin": [],
    "quota": "default",
    "storageLocation": "\/Users\/lreschke\/Programming\/core\/data\/admin",
    "lastLogin": "1418057287",
    "backend": "OC_User_Database"
}, {
    "name": "test",
    "displayname": "test",
    "groups": [],
    "subadmin": [],
    "quota": "default",
    "storageLocation": "\/Users\/lreschke\/Programming\/core\/data\/test",
    "lastLogin": 0,
    "backend": "OC_User_Database"
}]
```

@MorrisJobke Please review.