Allow clients to mark a node as in maintenance

Before the maintenance attribute was treated as an internal one, but it's
not, users should be able to mark a node as in maintenance. So this patch
is removing the maintenance attribute from the internal attributes list.

Change-Id: I35cbca53b7a0c253d2880bb7d350f70d5b4103a4
Merge "Allow clients to mark a node as in maintenance"