Restoring xtrace state for os-brick plugin

The change Ibb7423d243d57852dada0b6298463bbdfc6dc63c that introduced the
os-brick plugin introduced a flaw where the xtrace state wasn't restored
after the end of the plugin's execution. The end behavior is that devstack's
logs were with way less information, difficulting the debugging of the build.
This patch fixes the variable that was intended to hold the xtrace state (it
was using cinder's) and restoring the state at the end of the script.

Change-Id: I47c6c794a9704049b089142eca5603d1183f8a10

lib/os_brick

@@ -13,7 +13,7 @@
 # - install_os_brick
 
 # Save trace setting
-_XTRACE_CINDER=$(set +o | grep xtrace)
+_XTRACE_OS_BRICK=$(set +o | grep xtrace)
 set +o xtrace
 
 
@@ -27,3 +27,6 @@ function install_os_brick {
         setup_dev_lib "os-brick"
     fi
 }
+
+# Restore xtrace
+$_XTRACE_OS_BRICK