Add ProxyUrl parameter for OpenStack builder.

In my particular case I need to use a separate HTTP proxy to access my
OpenStack API than what is set in the environment. This commit adds an
optional parameter to the builder configuration and overrides the
net/http client that is used by the gophercloud library.
Website: Update timeout to reflect real default.

The default in the actual code is 5m.
Merge pull request #637 from johnbellone/master

builder/openstack: add ProxyUrl parameter for OpenStack builder