Add management of router/switchs global vlans

This allows to manage the global device list of vlans.
Currently supports only cisco IOS devices.

This is as easy as:

Vlan {
device_url => "ssh://user:pass@switch.domain.com/"
}

vlan {
"200": description => "R&D";
"99": description => "Management";
}

The device_url conforms to the same specs as for the interface
type.

Signed-off-by: Brice Figureau <brice-puppet@daysofwonder.com>