Ensured that dynatree cookies get deleted after a session reset.

The session reset that happens after a LDAP user switches groups, clears out all the dynatree sandboxes. However, the cookies are left intact which cause an issue with the dynatrees, making them lose
some functionality. For e.g. the trees expand the nodes, but nothing happens when the user clicks on them.

https://bugzilla.redhat.com/show_bug.cgi?id=1102250
https://bugzilla.redhat.com/show_bug.cgi?id=1086468