Low: add new Iface-Bridge agent to manage bridge interfaces as HA services

I got rather tired to manually configure service networks on all cluster nodes
every single time I have to test/deploy with multiple networks and complex setups bond+vlans+bridges (or
various combinations of them).

Agents for network config allows me to simply do:

pcs resource create vmbridge Iface-Bridge bridge_name=vmbridge bridge_slaves=eth1 clone

and have bridges configured across all nodes and gain also network health check at the same time.

This is the first of 3 agents that I am sending out for review (Bridge, Bond, Vlans).

Since they are all very similar, once this one passes review, I can cleanup the others
and send them for inclusion.

Note that some networking features are tight with kernel version and kernel build options.