5.15 should deploy with Jira v2.1.1 but on a fresh install I don't see any Jira plugin at all in the UI. This issue can be observed on rc.test.mattermost.com with rc2 now deployed. I can also be reproduced when deploying this release using a SpinMint.
Steps: (using bot-server to repro)
Open the DM with yourself
Type `/bot-server spin https://releases.mattermost.com/5.15.0-rc2/mattermost-5.15.0-rc2-linux-amd64.tar.gz <your_url>`
Once the server is ready, create admin user and login
Open system console and view plugin list
Observed No Jira plugin is listed
Verify the new bundle (RC3) once it's built, it should contain 2.1.2