Add parameter for filtering nodes by maintenance mode

Boolean parameter 'maintenance' added to API for filtering
nodes by maintenance mode, example:
/nodes?maintenance=false
strutils.bool_from_string() used for converting 'associated'
and 'maintenance' parameters, accepted values now for True
('1', 't', 'true', 'on', 'y', 'yes'), for False
('0', 'f', 'false', 'off', 'n', 'no') case-insensitive.
Refactored dbapi method get_node_list with filters used for
_get_nodes_collection in nodes controller.

Partial-Bug: #1260099
Partial-Bug: #1271291
Change-Id: I01a62eb641316e890d191cf569bb3cf00b7619f2
Merge "Add parameter for filtering nodes by maintenance mode"