[zerigo] Fix Invalid Excon request warning

This was triggering the warning:
`[excon][WARNING] Invalid Excon request keys: :host`

Followed by a stacktrace. Looking at [fog#2292][1] and the related
issues this seems like the right fix and seems to work fine.

[1]: https://github.com/fog/fog/pull/2292
Merge pull request #2899 from philk/zerigo_host_warning

[zerigo] Fix Invalid Excon request warning