Changes:
- signing tags.
- allows to override BRANCH and REPOSITORY using env variables.
Tested by a release in my private fork:
BRANCH="20201126-ptabor-release" REPOSITORY="git@github.com:ptabor/etcd.git" ./scripts/release 3.5.0-alpha.20
Examplar invocations:
Edit go.mod files such that all etcd modules are pointing on given version:
```
% DRY_RUN=false TARGET_VERSION="v3.5.13" ./scripts/release_mod.sh update_versions
```
Tag latest commit with current version number for all the modules and push upstream:
```
% DRY_RUN=true REMOTE_REPO="origin" ./scripts/release_mod.sh push_mod_tags
```