Clean-up VTEP-specific state from the ConfigurationService

This patch removes VTEP-specific state from the odl-ovsdb
ConfigurationService class, and makes the VTEP method reentrant,
by providing a Node argument identifying the VTEP connection.
This allows the same ConfigurationService to be shared between
multiple VTEP connections.

Signed-off-by: Alex Bikfalvi <alex.bikfalvi@midokura.com>