(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.
I know this is really minor, but it's a good exercise for me to try and flesh these comments out to build up my muscles more than get the code perfect or something like that.

>  I understand your argument. I don't think it's entirely correct, however - changing this to a def would only affect when the meminfo file is read from disk, not when the memory situation is actually checked/enforced (that happens below in validate-memory-requirements!).

I think we're on the same page, though I'm not sure what "correct" is in this case.  For me, correct behavior would be puppetserver checking available memory at a well-defined point in the process lifecycle and that check being as accurate as possible.

With this in mind, I wanted the implementation to read `/proc/meminfo` as close (in time) to the calculation as possible, not cached then used later in time when available memory could have changed.  Caching the contents of the file then calculating later on increases the chance of inaccurate results, or am I misunderstanding?