NETVIRT-1076: refactor arrow anti-patterns

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>