New(robot): Environment variable TEST_PORT for setting test port

[rkubis]

Description

There is environment variable called TEST_MR_HOST for Robot Framework tests setting Model Registry host that can be used for testing Model Registry instance deployed anywhere, but there is not any environment variable for setting port of the instance for testing it.

This PR provides new environment variable called TEST_MR_PORT for setting test port of Robot Framework tests.

How Has This Been Tested?

1. Deploy Model Registry instance anywhere with publicly reachable host and port.
2. Run Robot Framework tests against it with setting TEST_MR_HOST and TEST_MR_PORT environment variable:
   TEST_MR_HOST=<model-registry-hostname> TEST_MR_PORT=<model-registry-port> robot test/robot/UserStory.robot

Merge criteria:

• The commits and have meaningful messages; the author will squash them after approval or will ask to merge with squash.
• Testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious).
• The developer has manually tested the changes and verified that the changes work

[tarilabs]

thank you @rkubis for your PR ! 👏 this is very helpful to make it configurable

[rareddy]

/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rareddy, rkubis

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[tarilabs]

/lgtm