Allow configuring Rserve host from WebWork settings

The location of the Rserve host is read from
$pg{specialPGEnvironmentVars}{Rserve}{host}, which can be set in
webwork2/conf/localOverrides.conf
Add graceful failure in WWk macros if Rserve host is not configured

As suggested by John Jones and Jason Aubrey: if the value of
`$pg{specialPGEnvironmentVars}{Rserve}{host}` has not been set, then
have all of the other subroutines do nothing and print the message
about the need to configure Rserve.