Use python platform.system for system ID (#1499)

* Rebase and merging using platform.system

Rebasing and merging using platform.system instead of uname -a.

* Add missing import platform statement

* Remove subprocess import

Remove ununsed import subprocess to make changes flak8 compliant

lib/spack/spack/platforms/darwin.py

-import subprocess
+import platform
 from spack.architecture import Platform, Target
 from spack.operating_systems.mac_os import MacOs
 
@@ -22,6 +22,4 @@ def __init__(self):
 
     @classmethod
     def detect(self):
-        platform = subprocess.Popen(['uname', '-a'], stdout=subprocess.PIPE)
-        platform, _ = platform.communicate()
-        return 'darwin' in platform.strip().lower()
+        return 'darwin' in platform.system().lower()

lib/spack/spack/platforms/linux.py

-import subprocess
 import platform
 from spack.architecture import Platform, Target
 from spack.operating_systems.linux_distro import LinuxDistro
@@ -27,6 +26,4 @@ def __init__(self):
 
     @classmethod
     def detect(self):
-        platform = subprocess.Popen(['uname', '-a'], stdout=subprocess.PIPE)
-        platform, _ = platform.communicate()
-        return 'linux' in platform.strip().lower()
+        return 'linux' in platform.system().lower()