Test: PR testing update

[noredistribution]

Change Summary

Updated PR testing instructions to read passwords from env var, this way we don't need to update the inventory.yml in two places and we just need to export an env var

Component(s) name

arista.cvp.*

Proposed changes

How to test

Test by following the updated instructions in the PR, after the install.sh script is run to fetch a specific PR, we only need to export LABPASSPHRASE and then we can run the molecule test or the desired playbook.

Checklist

User Checklist

• N/A

Repository Checklist

• [x ] My code has been rebased from devel before I start
• [x ] I have read the CONTRIBUTING document.
• [ ] My change requires a change to the documentation and documentation have been updated accordingly. (check the box if not applicable)
• [ ] I have updated molecule CI testing accordingly. (check the box if not applicable)