Add validation to --property-filter in v1 shell

Previously, using --property-filter with invalid arguments resulted in
a rather cryptic error message:

    $ glance image-list --property-filter name
    dictionary update sequence element #0 has length 1; 2 is required

Now, something which is human decipherable is printed:

    $ glance image-list --property-filter name
    Argument --property-filter requires properties in the format KEY=VALUE

Change-Id: I61d19894fd8864bdca2fa3f627da3c7eb1341c51

glanceclient/v1/shell.py

@@ -80,6 +80,10 @@ def do_image_list(gc, args):
 
     if args.properties:
         property_filter_items = [p.split('=', 1) for p in args.properties]
+        if any(len(pair) != 2 for pair in property_filter_items):
+            utils.exit('Argument --property-filter requires properties in the'
+                       ' format KEY=VALUE')
+
         filters['properties'] = dict(property_filter_items)
 
     kwargs = {'filters': filters}