Bug 6535: Fix feature config pusher warning

Bug 6535: Fix feature config pusher warning

The feature config pusher assumes config files installed by features
that have XML extension are CSS files and tries to parse them. On failure
it prints a warning but ignores it and moves on. Up till now we've only
had CSS XML files but we now have XML files related to the
clustered-app-config. To avoid the warning I added a check to see if the
root element is "snapshot" if JAXB parsing fails.

Change-Id: I877921afc13564f131f61a1eb8327db71d3638fe
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>