Use uwsgi binary from path

All these uwsgi invocations assume that the uwsgi binary is in the
same directory as their project binaries are installed into (probably
/usr/bin).  That may not be correct -- for example if using a packaged
uwsgi on Fedora the binary will live in /usr/sbin/uwsgi (not /usr/bin
where the project files from pip are).

Switch invocations to just find it in the path.

Change-Id: I298e3374e9c84e209ffcabbaaacda17f8df19f4f
diff --git a/lib/keystone b/lib/keystone
index 366e6c7..d2f72a3 100644
--- a/lib/keystone
+++ b/lib/keystone
@@ -523,7 +523,7 @@
         enable_apache_site keystone
         restart_apache_server
     else # uwsgi
-        run_process keystone "$KEYSTONE_BIN_DIR/uwsgi --procname-prefix keystone --ini $KEYSTONE_PUBLIC_UWSGI_CONF" ""
+        run_process keystone "$(which uwsgi) --procname-prefix keystone --ini $KEYSTONE_PUBLIC_UWSGI_CONF" ""
     fi
 
     echo "Waiting for keystone to start..."