Remove use of Puppet::Util::Cacher in Puppet::SSL::Host

This class was previously using a cached_attr for its 'localhost' attribute,
representing the Puppet::SSL::Host entry corresponding to the cert in
Puppet[:certname]. We now no longer expire this attribute. This has the effect
that a change to certname during the lifetime of an agent will not be reflected
in the certificate it uses. If this behavior is desired, it will need to be
reimplemented another way.

Reviewed-By: Jacob Helwig <jacob@puppetlabs.com>