Fix PG counts and refactor FS creation during setup

Tests involving the filesystem are currently broken against ceph master, due to new limits on PGs per OSD.  Rather than just fiddling with the hardcoded values, I've rearranged where we do the creation a bit, so that there's a nice clean Filesystem.create that does the right thing.
@dachary in general CephManager has methods for operating on a configured ceph cluster, whereas the stuff for setting up a cluster lives elsewhere (mostly in tasks.ceph).  It felt weird to make write_conf a member of CephManager because of where it's used in tasks.ceph before the cluster is set up.  But it's an awkward thing because it's used both before and after the cluster is set up, so this transplanting it here is a bit of a compromise, so that both tasks.cephfs.filesystem and tasks.ceph can see it.