This adds support for Juju's storage hooks by merging the config
provided osd-devices with Juju storage provided osd-devices, in the
same way that the existing Ceph charm handles them.

In addition to providing support for ceph-osds via Juju storage,
we provide support for multiple journal devices through Juju storage
as well.

We have to add a shim hook to ensure that Ceph is installed prior
to storage hook invocation because storage attached at deploy time
will execute hooks before the install hook

Change-Id: Idad46e8f4cc32e09fbd64d29cd93745662e9f542