Provide gem_source attribute for fetching any required gems

[smford22]

Right now the source for where we fetch the inspec gem is hardcoded to rubygems.org. Some customers cannot access rubygems.org due to network restrictions. We should provide a gem_source attribute to override the source. The chef-vault cookbook provides this override as well...

https://github.com/chef-cookbooks/chef-vault

default['chef-vault']['gem_source'] = nil

chef_gem 'chef-vault' do # ~FC009
23      source node['chef-vault']['gem_source']
24      version node['chef-vault']['version']
25      clear_sources true unless node['chef-vault']['gem_source'].nil?
26      compile_time true
27    end

[chris-rock]

similar to #8

[chris-rock]

InSpec is part of Chef 13. If this is still required, please re-open