add config for caching metadata in memory

* add config metadata_memory_cache_ttl to enable caching metadata
in memory
* set default to 5 as project neutron/master

Conflicts:
	manifests/agents/metadata.pp

Change-Id: Ib6a05ae5033c53c4196f736373752370012d82a6
(cherry picked from commit 7babb4f4177908c5d5d122ff8bcd5a28ad5b38c7)