mgr: push localized config handling into BaseMgr*Module

mgr: push localized config handling into BaseMgr*Module

We need both (1) the raw key name (in order to look up the correct option
name and transform the result into the correct type) and (2) the optional
prefix (in order to look up both possible keys).  This simplifies the
mgr_module implementation.

There is a weird mismatch between the way that BaseMgrModule and
BaseMgrStandbyModule implement this method.  I suspect they could be
unified to work the same, but I'll leave that for another day.

Signed-off-by: Sage Weil <sage@redhat.com>