Json scheme for disks
Add validate_scheme method
Implement disks validation for api
Fix tests for disks
Add tests for validator
Remove 'No disk metadata specified for node' exception.
From VolumeManager constructor. Volume manager should not raise
such exceptions because it may occurs errors when node is provisioning
firstly.
Implement before deployment checking of minimal space for installation.
* now it depends on role of node.
* add tests, now we are tests disk size checking for all roles

Conflicts:

	nailgun/nailgun/test/test_node_disks.py
pep fixes
Extract helpers for retrieving disks or volume groups from spaces to volumes/manager.
[volumes] use only_disks helper and simplify logic.
Comment fixes for volume manager.
* fix comment
* add more verbose comment for calc_swap_size
Fix test for disks.
* test_recalculates_vg_sizes_when_disks_volumes_size_update
  We size of disk is 0, size of volumes in such disk should equal 0
Extract scheme validator to base validator class
Rename s/scheme/schema/
Fix unit-test for checking disks before deploying cluster.
DisksFormatConvertor inherited from object