doc(upgrade): upgrade instruction for nfs-provisioner

[mynktl]

Why is this PR required? What issue does it fix?:

What this PR does?:
This PR adds upgrade instruction for nfs-provisioner and nfs-server deployments.

Does this PR require any upgrade changes?: No

If the changes in this PR are manually verified, list down the scenarios covered::

Any additional information for your reviewer? :
Mention if this PR is part of any design or a continuation of previous PRs

Checklist:

• Fixes #
• PR Title follows the convention of <type>(<scope>): <subject>
• Has the change log section been updated?
• Commit has unit tests
• Commit has integration tests
• (Optional) Does this PR change require updating NFS-Provisioner Chart? If yes, mention the Helm Chart PR #
• (Optional) Are upgrade changes included in this PR? If not, mention the issue/PR to track:
• (Optional) If documentation changes are required, which issue on https://github.com/openebs/openebs-docs is used to track them:

[codecov-commenter]

Codecov Report

Merging #123 (2c18396) into develop (1ae6401) will decrease coverage by 0.09%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #123      +/-   ##
===========================================
- Coverage    49.88%   49.79%   -0.10%     
===========================================
  Files           38       38              
  Lines         2700     2705       +5     
===========================================
  Hits          1347     1347              
- Misses        1250     1255       +5     
  Partials       103      103              

Impacted Files	Coverage Δ
provisioner/config.go	9.90% <0.00%> (-2.30%)	⬇️
provisioner/provisioner.go	0.00% <0.00%> (ø)
provisioner/env.go	33.33% <0.00%> (+3.33%)	⬆️

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[mittachaitu]

Provided few comments

[mittachaitu]

LGTM

[kmova]

What happens in the following case:

• There are 2 nfs server deployments
• First run successfully upgraded 1 of them to latest images and script exited due to network issues or some other case
• User tries to upgrade again the remaining ones
• Will the first one be skipped or does the patch command return an error and not continue with upgrading next nfs server.

Also, can we have an optional parameter that says if user should upgrade only one PVC instead of all PVCs..

[mynktl]

In case of a partial upgrade, the second execution of the upgrade script will skip the upgrade for already upgraded deployment and proceed with the upgrade for non-upgraded nfs-server deployments.

I've updated the script to upgrade specific nfs-server deployment. Also updated the upgrade document with the command for the same.