rrd::cache: Added restrict_writes parameter

Added the '$restrict_writes' parameter which is false by
default. It configures the '-B' option to rrdcached.

« Only permit writes into the base directory specified in -b (and any
sub-directories).  This does NOT detect symbolic links.  Paths
containing "../" will also be blocked. »
Merge pull request #1 from sbadia/restrict_writes

rrd::cache: Added restrict_writes parameter