Operator can watch a configurable set of namespaces

[amuraru]

Change log description

This patch extends the watch mechanism in operator so the ZookeeperCluster resources can be deployed in different namespaces than where the operator runs.

Purpose of the change

Fixes #142

How to verify it

(Steps to verify that the changes are effective)

[codecov-commenter]

Codecov Report

Merging #201 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #201   +/-   ##
=======================================
  Coverage   82.82%   82.82%           
=======================================
  Files          11       11           
  Lines        1223     1223           
=======================================
  Hits         1013     1013           
  Misses        142      142           
  Partials       68       68           

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[anishakj]

I have tested below scenarios

1. Created operator in WATCH_NAMESPACE "" and created clusters in default and non default name spaces
2. Created operator in WATCH_NAMESPACE "ns1, ns2" and created clusters in ns1 and ns2
3. Created operator in WATCH_NAMESPACE "dev" and created cluster in dev namespace
   All scenarios have worked

[anishakj]

LGTM