Newer OneGate client broken with old OneGate servers

[vholer]

Description

If new OneGate client (e.g., 5.9.80) is running against old OneGate server (e.g., 5.4.15), the update operation fails:

# onegate vm update --data key=value
ERROR: 
[one.vm.update] Cannot update template. Parse error: syntax error, unexpected EQUAL, expecting $end at line 0, columns 22:23

Related to changes introduced by #1414, because format of data sent from client has changed:
https://github.com/OpenNebula/one/pull/3252/files#diff-25cc1e51ebff40297b9dd487dd43be4dR596

To Reproduce
Have old OneGate server and new OneGate client. Do update operation from inside VM.

Details

• Affected Component: OneGate
• Version: 5.9.80 (client), <5.9.80 (server)

Progress Status

• Branch created
• Code committed to development branch
• Testing - QA
• Documentation
• Release notes - resolved issues, compatibility, known issues
• Code committed to upstream release/hotfix branches
• Documentation committed to upstream release/hotfix branches

[al3xhh]

PRs to merge in master:

code: #3643
docs: OpenNebula/docs#732
context: OpenNebula/addon-context-linux#184

[vholer]

LGTM.

• Context merged
• Docs probably not necessary (this was a fix of feature introduced in master)
• Pending just code PR: B #3572: make onegate client compatible with old servers #3643