ceph-dev-new-trigger: only use positive match for filter

ceph-dev-new-trigger: only use positive match for filter

there is no need to put
```
grep -v '\(jewel\|kraken\|luminous\)' | grep '\(mimic\|nautilus\)'
```

as we are not likely to have both "jewel" and "mimic" in a branch name.

this change is a follow-up of the discussion when reviewing #1345

Signed-off-by: Kefu Chai <kchai@redhat.com>