tasks/ceph: conservative PG count in FS pools

This was tripping over the recent commit 42c85e80
in Ceph master, which tightens the limits on
acceptable PG counts per OSD, and was making
teuthology runs fail due to never going clean.

Rather than put in a new hardcoded count, infer
it from config.  Move some code around so that
the ceph task can get at a Filesystem object
to use in FS setup (this already has conf-getting
methods).

Signed-off-by: John Spray <john.spray@redhat.com>
tasks/ceph: move FS creation inside Filesystem

Leave the legacy handling out in cephfs_setup, move
the filesystem creation stuff into Filesystem.  I
anticipate this being the right place for it if/when
we have tests that want to do 'fs rm' 'fs new' type
cycles within themselves.

Signed-off-by: John Spray <john.spray@redhat.com>
Merge pull request #257 from ceph/wip-fs-creation

Fix PG counts and refactor FS creation during setup

Reviewed-by: Loic Dachary <ldachary@redhat.com>