Agent does not set ssh key from http metadata correctly on openstack

[fmoehler]

On openstack the agent does not correctly inject ssh keys from the http metadata service, even if any other settings source does not exist/doesn't return an ssh key. This seems to be due to this code:

bosh-agent/infrastructure/file_settings_source.go

Line 38 in 35ba0f3

return "", nil

where just an empty key is returned if the file settings source is set. If I compare the configurations between openstack and e.g. aws we see that aws only has the http metadata source defined, while openstack provides three methods.

I think either:

1. the file system source should return an error so we check the next source
2. we remove the file source from the stemcell builder
3. We could also change the order of the sources in the stemcell builder.

I proposed a potential fix (option 1) in the agent here: #437

Thanks for any input in advance.