Return the ability of upgrade from feature build

Problem:

Upgrading to feature builds was removed sometimes ago, but this ability was really helpful for testing fixes for upgrade.

Solution:

1. Setup pmm-server latest released version or an older version
2. navigate to https://pmm.cd.percona.com/job/pmm2-server-autobuild/build?delay=0sec and select repo as laboratory and provide the branch name the one from the submodules/FB branch we are testing, let it build and push pmm-server artifacts to the `laboratory` repo.
3. go inside pmm-server and enable laboratory repo by running:
   sed -i'' -e 's:/release/:/laboratory/:' /etc/yum.repos.d/percona-original-release.repo
4. click on the button `check for upgrades`
5. pmm-server should show a new update available, which will be the one pushed from this custom FB submodules branch
6. check upgrade to this FB pmm-server