fix(travis): add suffix to single arch image pushed by travis

[shubham14bajpai]

Signed-off-by: shubham shubham.bajpai@mayadata.io

What this PR does?: This PR adds the suffix amd64 to the cstor base image built in travis to avoid pushing on the multi-arch builds in action.

Checklist:

• [ ] Fixes #
• [ ] PR Title follows the convention of <type>(<scope>): <subject>
• [ ] Has the change log section been updated?
• [ ] Commit has unit tests
• [ ] Commit has integration tests
• [ ] (Optional) Are upgrade changes included in this PR? If not, mention the issue/PR to track:
• [ ] (Optional) If documentation changes are required, which issue on https://github.com/openebs/openebs-docs is used to track them: