Assigned self.front_os, and back_os to self.default. Helps with testing. If we're on a darwin or linux machine, chances are there is no back end or front end operating system, but rather than have those as None, I just assign them the default which is detected via python platform.

lib/spack/spack/platforms/darwin.py

@@ -12,7 +12,11 @@ def __init__(self):
         super(Darwin, self).__init__('darwin')
         self.add_target(self.default, Target(self.default))
         mac_os = MacOsx()
+        
         self.default_os = mac_os.name
+        self.front_os = mac_os.name
+        self.back_os =  mac_os.name 
+
         self.add_operating_system(mac_os.name, mac_os)
 
     @classmethod

lib/spack/spack/platforms/linux.py

@@ -13,6 +13,8 @@ def __init__(self):
         self.add_target(self.default, Target(self.default))
         linux_dist = LinuxDistro()
         self.default_os = linux_dist.name
+        self.front_os = linux_dist.name
+        self.back_os = linux_dist.name
         self.add_operating_system(linux_dist.name, linux_dist)
 
     @classmethod