PEX: add /etc/pexrc to the list of pexrc locations to check.

This allows administrators to make it more likely that pexes are executed using a "clean" python instead of whatever users have installed on their path. /etc/pexrc is the first rc read so that it has the lowest precedence and determined users can still override whatever they like.

Testing Done:
Manually tested /etc/pexrc.  I don't think the bang:buck is there for a unit test. (mocking out /etc takes work and there are already a bunch of other pantsrc locations that get tested anyway)

https://travis-ci.org/pantsbuild/pex/builds/91981161

Bugs closed: 183

Reviewed at https://rbcommons.com/s/twitter/r/3148/

Author: landism

pex/variables.py

@@ -52,7 +52,7 @@ def set(self, variable, value):
 
   def _from_rc(self, rc):
     ret_vars = {}
-    for filename in [rc, os.path.join(os.path.dirname(sys.argv[0]), '.pexrc')]:
+    for filename in ['/etc/pexrc', rc, os.path.join(os.path.dirname(sys.argv[0]), '.pexrc')]:
       try:
         with open(os.path.expanduser(filename)) as fh:
           rc_items = map(self._get_kv, fh)