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