Issue 185: support for imagepullsecrets field added

[Prabhaker24]

Change log description

Support for imagepullsecret added for allowing pull of images in a private repository.

Purpose of the change

Fixes #185

What the code does

imagepullsecret is added as part of pod policy and works with any other service account other than default and allows to do pull for images from a private repository

How to verify it

Following is tested and works:-

1. zookeeper cluster deployment with the zookeeper image in a private repo works for a non-default service account when correct imagepullsecret is specified.
2. zookeeper cluster deployment with the zookeeper image in public repository works for both default and non-default service account.

[codecov-io]

Codecov Report

Merging #272 (c439afe) into master (540aae6) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #272      +/-   ##
==========================================
+ Coverage   84.01%   84.07%   +0.06%     
==========================================
  Files          11       11              
  Lines        1201     1206       +5     
==========================================
+ Hits         1009     1014       +5     
  Misses        129      129              
  Partials       63       63              

Impacted Files	Coverage Δ
...g/apis/zookeeper/v1beta1/zookeepercluster_types.go	98.46% <ø> (ø)
...kg/apis/zookeeper/v1beta1/zz_generated.deepcopy.go	100.00% <100.00%> (ø)
pkg/zk/generators.go	99.22% <100.00%> (+<0.01%)	⬆️

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[anishakj]

LGTM

[SrishT]

LGTM