Use contemporary $() instead of legacy `` in shell commands

The "$()" form of command substitution solves a problem of inconsistent
behavior when using backquotes and is recommended over the legacy
backticks.

Change-Id: I7f0df0535822b64680b6c076e930814417bf1480
Signed-off-by: Joakim Roubert <joakimr@axis.com>

Makefile

@@ -2,7 +2,7 @@ TARGETS := $(shell ls scripts | grep -v \\.sh)
 
 .dapper:
 	@echo Downloading dapper
-	@curl -sL https://releases.rancher.com/dapper/v0.4.2/dapper-`uname -s`-`uname -m` > .dapper.tmp
+	@curl -sL https://releases.rancher.com/dapper/v0.4.2/dapper-$(uname -s)-$(uname -m) > .dapper.tmp
 	@@chmod +x .dapper.tmp
 	@./.dapper.tmp -v
 	@mv .dapper.tmp .dapper

tests/perf/scripts/test

@@ -3,8 +3,8 @@
 test_load() {
   source scripts/config
   eval PRIVATE_KEY_PATH=$PRIVATE_KEY_PATH
-  EXPANDED_PRIV_KEY_PATH=`readlink -f $PRIVATE_KEY_PATH`
-  masterips=`terraform output -state=server/server.tfstate | grep k3s_server_ips | cut -d "=" -f 2`
+  EXPANDED_PRIV_KEY_PATH=$(readlink -f $PRIVATE_KEY_PATH)
+  masterips=$(terraform output -state=server/server.tfstate | grep k3s_server_ips | cut -d "=" -f 2)
   pushd tests/
   docker run -v $EXPANDED_PRIV_KEY_PATH:/opt/priv_key \
              -e KUBE_SSH_USER=ubuntu \
@@ -23,8 +23,8 @@ test_load() {
 test_density() {
   source scripts/config
   eval PRIVATE_KEY_PATH=$PRIVATE_KEY_PATH
-  EXPANDED_PRIV_KEY_PATH=`readlink -f $PRIVATE_KEY_PATH`
-  masterips=`terraform output -state=server/server.tfstate | grep k3s_server_ips | cut -d "=" -f 2`
+  EXPANDED_PRIV_KEY_PATH=$(readlink -f $PRIVATE_KEY_PATH)
+  masterips=$(terraform output -state=server/server.tfstate | grep k3s_server_ips | cut -d "=" -f 2)
   pushd tests/
   docker run -e KUBE_SSH_USER=ubuntu \
              -v $EXPANDED_PRIV_KEY_PATH:/opt/priv_key \