create different roles for dk webnode with and without AWS ElastiCache. they use different versions of the dk_webnode cookbook
use pecl-memcached and pin this as version 0.1.0 of the cookbook
move memcached recipe up in the run_list, as there are clashing versions of libmemcached and libmemcached-devel between the memcached recipe and the php::source recipe when it builds pecl-memcached from source. in this order the bootstrapping works correctly and the machine is left with up-to-date versions of the libs installed.