Fix install_venv-get_distro failure on Fedora

There was no check_exit_code=False used so install_venv died
on Fedora/RHEL/etc.
Also calling external grep command is not neccessary.

Change-Id: I280aed3019b8cb7fce67033158deb34937946fab
diff --git a/tools/install_venv.py b/tools/install_venv.py
index f4621c9..28275ba 100644
--- a/tools/install_venv.py
+++ b/tools/install_venv.py
@@ -150,16 +150,16 @@
 
 
 def get_distro():
-    if (os.path.exists('/etc/fedora-release') or
-            os.path.exists('/etc/redhat-release')):
-        if os.path.exists('/etc/redhat-release') \
-            and run_command_with_code(['grep', 'CentOS',
-                                       '/etc/redhat-release']) == 0:
-            return CentOS()
-        else:
-            return Fedora()
-    else:
-        return Distro()
+    if os.path.exists('/etc/redhat-release'):
+        with open('/etc/redhat-release') as rh_release:
+            if 'CentOS' in rh_release.read():
+                return CentOS()
+        return Fedora()
+
+    if os.path.exists('/etc/fedora-release'):
+        return Fedora()
+
+    return Distro()
 
 
 def check_dependencies():