Find custom scripts in path like Git does

People often make custom wrappers around their calls to OpenStack. It would be useful to be able to have these scripts treated like just more OpenStack commands. Eg, you have a script to convert an IP address into a server ID? You want to call `openstack ip2server` or something, not `neutron port-list --fixed-ips ip_address=<xxx.xxx.xxx.xxx> -F device_id -c device_id -f value`.

Looking to the precedent set by Git, it seems reasonable that we could put a script called `openstack-ip2server` (or whatever you want to call it) somewhere in our path and then openstack-client would find it.

This would work especially well when using a virtualenv. All your custom scripts can be symlinked into the virtualenv `bin`, and then `openstack ip2server` will be available if and only if the OpenStack virtualenv is currently active.