Avoid allocating empty strings in the Env slice

Previously, we declared the Env slice with the size of the `c.Services.KubeAPI.ExtraEnv` field, which leads to empty strings at the beginning of the Env slice because we use Golang's append function to add new elements, and Docker is not happy with that.
We fix this issue by declaring the Env variable as an empty slice. We also enhance the `getUniqStringList` function to properly trim leading and trailing spaces in each element and to ignore empty strings. We add unit tests for the `getUniqStringList` function and update the integration tests.

scripts/integration

@@ -90,6 +90,25 @@ nodes:
 - address: rke-node-${node}
   role: [etcd, controlplane, worker]
   user: ubuntu
+services:
+  etcd:
+    extra_env:
+      - TEST_VAR=etcd
+  kubeproxy:
+    extra_env:
+      - TEST_VAR=kube-proxy
+  scheduler:
+    extra_env:
+      - TEST_VAR=scheduler
+  kubelet:
+    extra_env:
+      - TEST_VAR=kubelet
+  kube-controller:
+    extra_env:
+      - TEST_VAR=kube-controller
+  kube-api:
+    extra_env:
+    - TEST_VAR=kube-api
 EOF
 
     if [ "x${NETWORK_PLUGIN}" != "x" ]; then