The env variable for system chart branch per release version will be set from package/Dockerfile only

We need the agent tag to be unique for master builds so that when
you upgrade you always get the proper agent image pushed to the
downstream clusters.

Agent image had the wrong version because of this mistake.
It is working correctly on v2.2 branch and this brings the two in line.

Added args and env vars to build files. These args are needed
to maintain proper system-chart branch for particular rancher
versions. This makes changes in support for make and drone.

- Change `POWERSHELL_IMG_TAG` to `SERVERCORE_VERSION` on `package.ps1`
- Add `SERVERCORE_VERSION` on `.drone.yml` when publishing Windows agent
image

**Problem:**
- Cannot set up kubelet on Windows
- Sometimes, host kubelet cannot connect controlplanes via container nginx proxy, but we found that only the connection between kubelet and container nginx has been broken.
- Upgraded from previous existing cluster causes Windows agent failed
- Running a few days, Windows host will crash by OOM, as `docker create
-> docker start` leaks none-page memory
- The Windows code doesn't look very clear
- Need to support Flannel vxlan mode
- Need to support CI
- Need to support Docker manifest images
- Need to add unit test

**Solution:**
- Adjust kubelet options
- Replace container Nginx proxy by host Nginx
- Fix Windows error when upgrading rancher
    + Use `AllK8sWindowsVersions` to index system images
    + Specify the `NetworkMode` of Process to `none`
- For Windows, replace `docker create -> docker start` to `docker run`
- Refactor Windows agent
    + Separate logrus hook into a file
    + Create `winRunner` to execute main logic
    + Remove ugly break label
    + Separate connecting into a single method
    + Only treat Powershell active retry exit status
    + Adjust PowerShell script
        + Redirect log to c:\var\log
        + Reduce retry count
        + Change log.ps interval to 1s
        + Log to file if running background
- Support Flannel Vxlan Mode
    + Adjust PowerShell script via moving `net-conf.json` into `start-flanneld`
    + Support to configure Flanneld backend via `FlannelBackendConfig`
- Support CI
    + Use dapper
    + Patch Linux pipelines with OS info
    + Add Windows 1809 Drone support
    + Add .gitattributes file to fix https://github.com/golang/go/issues/16355
- Support Docker manifest images
    + Remove the processing of Windows image suffix in everywhere
- Unit test
    + There is not syslog implemented on Windows, so separate by os
    + Add test logic into CI

**Issue:**
- https://github.com/rancher/rancher/issues/17341
- https://github.com/rancher/rancher/issues/16074
- https://github.com/rancher/rancher/issues/19048
- https://github.com/rancher/rancher/issues/17499
- https://github.com/rancher/rancher/issues/17499

1. Update drone1 config file for multiarch CICD
2. Update dapper and ci scripts for multiarch
3. Update test_kontainer_drivers.py for arm64 platform pytest.