vyatta-openvpn: formatting changes for style consistency

Update lib/Vyatta/OpenVPN/Config.pm to have consistent identation
levels and style throughout.
vyatta-openvpn: add 'set interfaces openvpn vtun0 persistent-tunnel'

Add the option to make OpenVPN interfaces (TUN/TAP device) persistent
across client restarts.  This patch expands on the previous fix for
Bug #7 and Bug #313, where the interface description was maintained
for 'show interfaces openvpn', but ifAlias was unset and ifIndex
changed, following a client reset.