Check cluster_node.is_managed before setting node as provisioner

[r0h4n]

"Provisioner" is a node elected per gluster cluster by Tendrl, it seems currently when new un_managed nodes are lined up for expansion of cluster, they also become eligible for "Provisioner" tag election.

New un_managed nodes should not be part of "Provisioner" tag elections.

eg: Unmanaged node became "Provisioner"

{"action":"get","node":{"key":"/clusters/48558a4f-f8a0-49b4-bd2c-51878bf60b7d/nodes/c8cae4fb-841d-49bf-ad2b-e6ee6fbe855c/NodeContext","dir":true,"nodes":[{"key":"/clusters/48558a4f-f8a0-49b4-bd2c-51878bf60b7d/nodes/c8cae4fb-841d-49bf-ad2b-e6ee6fbe855c/NodeContext/updated_at","value":"2018-07-11 14:48:03.482988+00:00","modifiedIndex":36850,"createdIndex":36850},{"key":"/clusters/48558a4f-f8a0-49b4-bd2c-51878bf60b7d/nodes/c8cae4fb-841d-49bf-ad2b-e6ee6fbe855c/NodeContext/data","value":"{\"integration_id\": \"\", \"status\": \"UP\", \"tags\": \"[\\\"detected_cluster/7d0f4a7727c9c8f7e9e8f6b055f2fe14f51fc18dac8b450ab9ea4cc8d4a971b5\\\", \\\"tendrl/node_c8cae4fb-841d-49bf-ad2b-e6ee6fbe855c\\\", \\\"tendrl/integration/48558a4f-f8a0-49b4-bd2c-51878bf60b7d\\\", \\\"gluster/server\\\", \\\"provisioner/48558a4f-f8a0-49b4-bd2c-51878bf60b7d\\\", \\\"tendrl/node\\\"]\", \"sync_status\": \"done\", \"fqdn\": \"fbalak-usm2-gl5.usmqe.lab.eng.brq.redhat.com\", \"last_sync\": \"2018-07-11 14:31:29.162987+00:00\", \"first_sync_done\": \"\", \"node_id\": \"c8cae4fb-841d-49bf-ad2b-e6ee6fbe855c\", \"ipv4_addr\": \"10.37.169.132\", \"is_managed\": \"\"}","modifiedIndex":36849,"createdIndex":36849},{"key":"/clusters/48558a4f-f8a0-49b4-bd2c-51878bf60b7d/nodes/c8cae4fb-841d-49bf-ad2b-e6ee6fbe855c/NodeContext/hash","value":"4452af6e6f2201d14fd2f7e66fe594fc","modifiedIndex":36851,"createdIndex":36851},{"key":"/clusters/48558a4f-f8a0-49b4-bd2c-51878bf60b7d/nodes/c8cae4fb-841d-49bf-ad2b-e6ee6fbe855c/NodeContext/status","value":"UP","modifiedIndex":2800755,"createdIndex":2800755}],"modifiedIndex":22774,"createdIndex":22774}}