Improved conda configuration for our plumed channel

Now it works like this:
- test- branches and pull requests are built with conda but not uploaded.
- tags are built with conda and uploaded with `tags` label.
- cron builds are built with conda and uploaded with `nightly` label.

Nightly builds are assignd a version number equal to branch.date
for version branches or 2.99.date for master branch, e.g.

2.5.190603
2.99.190603

In this way they always supersede the corresponding releases.

See #412