Move non-bridge-related OVSBridge methods to BaseOVS

Move non-bridge-related OVSBridge methods to BaseOVS

This patch moves several methods in OVSBridge that don't really have
anything to do with OVS bridges to BaseOVS where they are a much
better fit. Since OVSBridge inherits from BaseOVS, no modules using
ovs_lib will have to change to support this move.

Also, several module-level functions that essentially re-implement
BaseOVS.run_vsctl() are moved to BaseOVS and converted to use
run_vsctl. In order to avoid changing the API, the module-level
functions are then reimplemented by creating a BaseOVS instance and
calling the associated method.

Partially-implements: blueprint vsctl-to-ovsdb
Change-Id: I94aaa0bef68e407649b421db53bb59d007335f5c