Add attribute firewall_policy to nic
Add ready? to server class
Mark servers collection tests as pending
Changes to rely on fog-core

This references `fog-core` and removes the files it now contains.
[google|compute] reraise LoadError when google-api-client missing
Merge pull request #2659 from dJason/patch-1

[cloudsigma] adding firewall_policy attribute and ready? method for servers
Merge pull request #2663 from fog/use_fog_core

Changes to rely on fog-core
add fog-core to 1.8.7 Gemfile also