[libcalamares] Move CommandList into libcalamares

- Move CommandList so it can be used from more modules than
   just ShellProcess
 - Allow a CommandList to run itself. This centralizes
   code for executing one or more commands and simplifies
   the ShellProcess module.

Various small cleanups:
 - mention instance id in log message
 - code formatting / style

src/modules/shellprocess/Tests.cpp

@@ -17,8 +17,8 @@
  */
 
 #include "Tests.h"
-#include "CommandList.h"
 
+#include "utils/CommandList.h"
 #include "utils/YamlUtils.h"
 
 #include <yaml-cpp/yaml.h>
@@ -30,6 +30,8 @@
 
 QTEST_GUILESS_MAIN( ShellProcessTests )
 
+using CommandList = CalamaresUtils::CommandList;
+
 ShellProcessTests::ShellProcessTests()
 {
 }