add auto creation of ovs bridge

Allow auto-creation of an ovs bridge (somewhat inline with auto-creation of a linux bridge). Throw an error if 'ovs*' is specified as a bridge and ovs is not installed.
Thanks for reviewing @jpetazzo. Few responses to your comments.

1)
The reason for keeping it earlier was to avoid any partial changes to the system. For example, following (and potentially future changes) would have run if the check was put inlater, unless we add cleanup code during failures.
[ ! -d /var/run/netns ] && mkdir -p /var/run/netns
[ -f /var/run/netns/$NSPID ] && rm -f /var/run/netns/$NSPID
ln -s /proc/$NSPID/ns/net /var/run/netns/$NSPID

2) 
I did test with ovs-tools and utilities not installed, etc.. and it reports an error, as expected. OK so that's basic stuff. however the situation you are describing when ovs bridge is created by another container and the native tools are not installed on the host then IMHO all ovs-operations (i.e. pipework) should be called from within container itself. In that case the changes would work.

Still, if you feel strongly that the advantage in 1) outweighs any potential disadvantages of 2), I will move the code down during bridge creation.