linstor: use relative hostname path

As described in issue apache#8310 some older distributions don't have
hostname in /usr/bin so rely on PATH resolving

plugins/storage/volume/linstor/src/main/java/com/cloud/hypervisor/kvm/storage/LinstorStorageAdaptor.java

@@ -72,7 +72,7 @@ private String getLinstorRscName(String name) {
 
     private String getHostname() {
         // either there is already some function for that in the agent or a better way.
-        ProcessBuilder pb = new ProcessBuilder("/usr/bin/hostname");
+        ProcessBuilder pb = new ProcessBuilder("hostname");
         try
         {
             String result;
@@ -88,7 +88,7 @@ private String getHostname() {
             return result.trim();
         } catch (IOException | InterruptedException exc) {
             Thread.currentThread().interrupt();
-            throw new CloudRuntimeException("Unable to run '/usr/bin/hostname' command.");
+            throw new CloudRuntimeException("Unable to run 'hostname' command.");
         }
     }