(SERVER-259) Improve validate-memory-requirements! error message

Without this patch the fatal error message given to the user when RAM is less
than 1.1 * JVM heap size is the following following:

    Not enough RAM. Puppet Server requires at least 2181MB of RAM.

This is a problem because it leads the user to believe they don't have enough
RAM.  They may conclude their only recourse is to add more memory when another
recourse is to decrease the configured maximum JVM heap size.

This patch addresses the problem by augmenting the error message to
specifically call out the JVM heap setting as a possible recourse.  The message
further tries to help the user by directing them to the JAVA_ARGS, -Xms, -Xmx
options, and the locations of the configuration files on Debian and Enterprise
Linux, the most common platforms.
In the rest of the codebase, instead of using a dynamic var for testing purposes, we use `with-redefs` in the test.  I think that is a better approach because it does not have an affect on the production code.