LCGDM-1401 allow disabling memcached

if the memcache config class is called with enable_memcache == false,
it will now remove all config files
create convenience class to configure dpm with adapter only

new class head_adapter.pp, which can be used instead of head.pp
to configure the dpm with only the adapter. it enables the
necessary adapter plugins (Ns and Dpm) and ensures the mysql
config file is not present.
add the dpm_mmrqhost variable in the federation config