Check cluster_node.is_managed before setting node as provisioner

[r0h4n]

tendrl-bug-id: #840

bugzilla: 1599634

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@0768af2). Click here to learn what that means.
The diff coverage is 0%.

@@            Coverage Diff            @@
##             master     #841   +/-   ##
=========================================
  Coverage          ?   42.26%           
=========================================
  Files             ?       72           
  Lines             ?     3012           
  Branches          ?      446           
=========================================
  Hits              ?     1273           
  Misses            ?     1676           
  Partials          ?       63

Impacted Files	Coverage Δ
...rl/node_agent/node_sync/services_and_index_sync.py	10.46% <0%> (ø)

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