In general: added location fields to device_info model.

    Details:
    Added models to ``admin``:
    - data_center
    - server_room
    - rack

    Updated ``clean`` method on ``device_info`` model, so that field constaraints
    are respected (see docstring of ``clean``)
    Added new form field to dc asset: hostname.
    Fixed tests and added new ones.
Change AssetStatus.used status description to more readable
fix: updated DeviceEnvironment name to DeviceEnvironmentLookup
Merge pull request #517 from xliiv/lookup-fix

fix: updated DeviceEnvironment name to DeviceEnvironmentLookup
Merge pull request #518 from xliiv/location-form-fields-ok

added location fields to device_info form
added rack, data center, server room to search conditions
Entrypoints for visualization datacenter

- Added entrypoints as api. Using django rest framework
- Created new place named `rest` for all django rest framework stuff
- Added tests and routing insite `rest` folder
Merge pull request #516 from quamilek/change-status-name

Change AssetStatus.used status description to more readable
fix: updated tests after factories update
Merge pull request #1 from xliiv/arek-rack-search

fix: updated tests after factories update
Merge branch 'develop' into feature/prepare-endpoints-for-racks

Conflicts:
	src/ralph_assets/tests/utils/assets.py
Merge pull request #507 from ar4s/70-rack-in-search-form

Added rack, data center, server room to search conditions
Many commits: because travis has trouble with building.
Merge pull request #515 from kula1922/feature/prepare-endpoints-for-racks

Entrypoints for visualization datacenter
Added without_assisgned_location field to dc-search form.
Enables to search dc-assets without location fields.
Location fields are: data_center, server_room, rack, etc.
Merge pull request #520 from xliiv/no-location-in-search-ok

Added without_assisgned_location field to dc-search form.