move calls to cstdlib::getenv to env::getEnv

Wrapped the cstdlib::getenv() call into env::getEnv
so that it can be tested by mocking the env namespace,
by just dropping in test_env.cpp which implements those
methods and tying them a singleton upon which we can set
expectations.

Note: regardless of the approach taken to mock this, wrapping
this method is a good fix.

Change-Id: I65d22da08fc3dd76e6860f728c54e6c847ed07de
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/env.cpp b/env.cpp
index 09dd985..6841987 100644
--- a/env.cpp
+++ b/env.cpp
@@ -22,23 +22,23 @@
 
 namespace env {
 
+std::string getEnv(const char* key)
+{
+    auto value = std::getenv(key);
+    return (value) ? std::string(value) : std::string();
+}
+
 std::string getEnv(
     const char* prefix, const SensorSet::key_type& sensor)
 {
     std::string key;
-    std::string value;
 
     key.assign(prefix);
     key.append(1, '_');
     key.append(sensor.first);
     key.append(sensor.second);
-    auto env = getenv(key.c_str());
-    if (env)
-    {
-        value.assign(env);
-    }
 
-    return value;
+    return getEnv(key.c_str());
 }
 
 std::string getEnv(