Add enable_tunneling openvswitch configuration variable

Add enable_tunneling openvswitch configuration variable

Not all systems that support Open vSwitch support its GRE tunneling
feature that is not in the Linux kernel source tree. Therefore, a new
configuration variable, enable_tunneling, applies to both the server
and agent. Its default value is False, so it must be set to True to
enable tunneling for tenant and/or provider networks. If
enable_tunneling is False, the server will not allow creation of GRE
networks, and the agent will not initialize the tunnel bridge and will
log an error if there is an attempt to provision a GRE network. If it
is True, the agent now logs an error and exits if it fails to
initialize the patch ports used for the tunnel bridge. Fixes bug
1045610.

When there is an attempt to provision a flat or VLAN network and the
agent has no bridge mapping configured for a the specified physical
network, the agent will now log an error rather than crash.

An undefined variable in a logging statement has been corrected.

Logging levels for openvswitch have been checked and updated where
necessary, and logging statements now avoid the % string substitution
syntax. Fixes bug 1045592.

The corresponding devstack update has already been merged.

Change-Id: I149db182dd132cc05802dcb20c6b552e293664a5