fix(python conversation): Fix timestamp and context properties

[ehdsouza]

We had the created & updated properties still marked as required.
When we were merging the request & response models for Context, we left out the conversation_id and system to be required which is not true when in a request model but is valid on response model.
This fixes the above to two issues by making the properties to be optional so that the SDK

[codecov-io]

Codecov Report

Merging #414 into develop will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           develop    #414   +/-   ##
=======================================
  Coverage        0%      0%           
=======================================
  Files           22      22           
  Lines        12997   13001    +4     
=======================================
- Misses       12997   13001    +4

Impacted Files	Coverage Δ
watson_developer_cloud/conversation_v1.py	0% <0%> (ø)	⬆️

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[anweshan]

This looks good to me