Add option to enable and set package manager proxy url

With this feature you can set the proxy url for the package manager.
Currently supports Yum and Apt package managers.
Support custom hypervisors
Merge pull request #247 from JHaals/hypervisor

Support custom hypervisors
Removing unnecessary `razor` wrapper function

The `razor` function was unnecessarily defined in the
wrapper, which was overriding a helper definition.
Merge pull request #294 from smcclellan/remove-unneeded-razor-method

Removing unnecessary `razor` wrapper function
Merge pull request #273 from electrical/feature/proxy

Add option to enable and set package manager proxy url