Build: Change build to always use system leveldb

Dynamically link to the leveldb installed on the system rather than
statically linking ceph copy.  Remove the --with-system-leveldb config
option, and add a requirement for leveldb libraries for rpm and debian
packages.  Bug 3945.

Signed-off-by: Gary Lowell  <gary.lowell@inktank.com>
Merge git://github.com/ceph/ceph