support purging for all types of resources

[MaxFedotov]

Hello,

right now in some resources we have

  validate do
    raise('name parameter is required.') if (self[:ensure] == :present) && self[:name].nil?
    raise('hostname parameter is required.') if (self[:ensure] == :present) && self[:hostname].nil?
    raise('port parameter is required.') if (self[:ensure] == :present) && self[:port].nil?
  end

so the error is raised only in case if the resource should be present, but for some, it is not true.
So let's make this equal for all resource types and support purging for all of them

[saz]

Fixes #161