agent: report Linux cached/buffers memory

Some applications, like Oracle Directory Server, use cached memory
and never release it. Users need to take this memory into account
when planning the memory requirements.

Engine counterpart tracked by bz#1024010.

Change-Id: Id46f85b94937779ea899b05dd1bfa73780a1a4e9
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1024007
Signed-off-by: Amador Pahim <apahim@redhat.com>