Reduce config access in scheduler

Caching the configuration appears to make a impact on the time spent
in the scheduler. These config values reduce the time spent down from
around 5ms to around 4ms per request, when with 200 hosts.

Change-Id: I433cbbdbef4ac3c01a3e2b6ed5cf092a5cce6372
Enable ServerGroup scheduler filters by default

The Icehouse release includes a server group REST API. For these
groups to actually function properly, the server group scheduler
filters must be enabled. So, these filters should be enabled by
default since the API is also enabled by default. If the API is not
used, the scheduler filters will be a no-op.

DocImpact
Change-Id: Ia52d7fcfff52377d8beae2acd9b7eba9c6c74e09
Closes-bug: #1303983
Merge "Reduce config access in scheduler"