(#19514)(PP-773) Rename hashed_node_data and cliencert for $trusted

This renames hashed_node_data to trusted_node_data, and
the element 'clientcert' to 'certname'.

This also fixes the issues regarding immutability reported in PP-773.
It is now not possible to alter the trusted data. (A freeze was missing
when the data was created). As extra precaution, the data is frozen when
set as trusted data in the node.

There is now also a warning if an attempt is made to modify the trusted
data by manipulating a node.
(#19514) Correct error message matching for Ruby 1.8.7

In Ruby 1.8.7, the error message for an attempt of mutating
a frozen hash refers to 'hash' using lower case whereas later versions
use capital 'H'. This caused two tests to fail (twice).
Merge pull request #2084 from hlindberg/issues/issue-19514

(#19514)(PP-773) Rename hashed_node_data and cliencert for $trusted