Added cache:key to .gitlab-ci.yml allowing to fine tune the caching
The `cache:key` allows you to define the affinity mask of caching, allowing to have single cache for all jobs, or cache per-job, or per-branch, or any other way you would need:
1. Cache per-build for all branches:
```
cache:
key: "$CI_BUILD_NAME"
untracked: true
```
2. Cache per-branch for all jobs:
```
cache:
key: "$CI_BUILD_REF"
untracked: true
```
/cc @DouweM @grzesiek @axil
See merge request !2436
54729d2c28