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

Without this patch the 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.