(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 took a stab at improving the message and including the value for total (available) system ram and how we go about calculating "required" memory.  The message now looks like:

`Not enough available RAM (1834MB) to safely accommodate the configured JVM heap size of 2048MB.  Puppet Server requires at least 2252MB of available RAM given this heap size, computed as 1.1 * max heap (-Xmx).  Either increase available memory or decrease the configured heap size by reducing the -Xms and -Xmx JAVA_ARGS settings in /etc/sysconfig/puppetserver on EL systems or /etc/default/puppetserver on Debian systems.`