Namespace XTRACE commands

I noticed this when debugging some grenade issues failures.

An include of grenade/functions stores the current value of XTRACE
(on) and disables xtrace for the rest of the import.

We then include devstack's "functions" library, which now overwrites
the stored value of XTRACE the current state; i.e. disabled.

When it finishes it restores the prior state (disabled), and then
grenade restores the same value of XTRACE (disabled).

The result is that xtrace is incorrectly disabled until the next time
it just happens to be turned on.

The solution is to name-space the store of the current-value of xtrace
so when we finish sourcing a file, we always restore the tracing value
to what it was when we entered.

Some files had already discovered this.  In general there is
inconsistency around the setting of the variable, and a lot of obvious
copy-paste.  This brings consistency across all files by using
_XTRACE_* prefixes for the sotre/restore of tracing values.

Change-Id: Iba7739eada5711d9c269cb4127fa712e9f961695

lib/nova_plugins/functions-libvirt

@@ -8,7 +8,7 @@
 # ``STACK_USER`` has to be defined
 
 # Save trace setting
-LV_XTRACE=$(set +o | grep xtrace)
+_XTRACE_NOVA_FN_LIBVIRT=$(set +o | grep xtrace)
 set +o xtrace
 
 # Defaults
@@ -134,7 +134,7 @@ EOF
 
 
 # Restore xtrace
-$LV_XTRACE
+$_XTRACE_NOVA_FN_LIBVIRT
 
 # Local variables:
 # mode: shell-script