NETVIRT-1076: refactor arrow anti-patterns
* NeutronvpnUtils:
** getSubnetMapsforNetworkRoute() method is replaced by
getPrivateSubnetsToExport(), which obtains the list of private
subnets from router in more simple way;
** addExternalNetworkToVpn() and removeExternalNetworkFromVpn() were
moved to NeutronvpnManager to avoid import collisions.
* NeutronvpnNatManager:
** use getPrivateSubnetsToExport() instead of
getSubnetMapsforNetworkRoute();
** fix log messages.
* NeutronvpnManager:
** refactor arrow anti-patterns in associateNetworksToVpn() and
dissociateNetworksFromVpn() methods;
** add a support of use-case, when BGPVPN can be associated with more
than one Provider Network;
** fix log messages;
JIRA: NETVIRT-1076
Change-Id: Ifea2e0d2c2631ac87a3883f5a59d5fd3f189f812
Signed-off-by: Valentina <valentina.krasnobaeva@6wind.com>