add compat symlink for kubelet volume plugins

[bcressey]

Issue number:
Fixes #1354

Description of changes:
This allows existing manifests that try to install a flexvol plugin to the default /usr/libexec/kubernetes path to work, rather than failing because the root filesystem is read-only.

Testing done:
Followed the steps to "Install EKS with Calico networking":
https://docs.projectcalico.org/getting-started/kubernetes/managed-public-cloud/eks

The symlink worked as expected from both the /usr path and the target prefix:

# ls -latr /usr/libexec/kubernetes/kubelet-plugins
lrwxrwxrwx. 1 root root 38 Mar 24 23:19 /usr/libexec/kubernetes/kubelet-plugins -> ../../../../../var/lib/kubelet/plugins

# ls -latr /usr/libexec/kubernetes/kubelet-plugins/
drwxr-xr-x. 3 root root 4096 Mar 24 23:55 volume
drwxr-xr-x. 2 root root 4096 Mar 24 23:56 ebs.csi.aws.com
drwxr-xr-x. 2 root root 4096 Mar 24 23:56 efs.csi.aws.com

# ls -latr /x86_64-bottlerocket-linux-gnu/sys-root/usr/libexec/kubernetes/kubelet-plugins
lrwxrwxrwx. 1 root root 38 Mar 24 23:19 /x86_64-bottlerocket-linux-gnu/sys-root/usr/libexec/kubernetes/kubelet-plugins -> ../../../../../var/lib/kubelet/plugins

# ls -latr /x86_64-bottlerocket-linux-gnu/sys-root/usr/libexec/kubernetes/kubelet-plugins/
drwxr-xr-x. 3 root root 4096 Mar 24 23:55 volume
drwxr-xr-x. 2 root root 4096 Mar 24 23:56 ebs.csi.aws.com
drwxr-xr-x. 2 root root 4096 Mar 24 23:56 efs.csi.aws.com

The node agent plugin showed up in the expected location:

# find /var/lib/kubelet/plugins -type s -o -type f
/var/lib/kubelet/plugins/efs.csi.aws.com/csi.sock
/var/lib/kubelet/plugins/volume/exec/nodeagent~uds/uds
/var/lib/kubelet/plugins/ebs.csi.aws.com/csi.sock

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[bcressey]

Fixed excessive use of ../.