Previously we only alterred the mdraid member flag to a disks role
when it's fstype, as reported by scan_disks, indicated the need.
However this was done within a conditional that precluded
examining a 'None' fstype which is what scan_disks translates an
empty string into. Hence we never updated a role db entry for a
previous mdraid member once that member no longer returned an
fstype. Resolved by moving the role label logic outside the
previous conditional such that it now applies to all disks even if
they return no fstype from scan_disks.

We need now to prove this works as expected
so add additional logging to help.

Previously envisioned as space separated values, update
to reflect proposed (by @suman) json format for this
field.

Also add beginnings of accounting for roles other than
mdraid member. Note this element is incomplete here.
N.B. provision is included for dealing with existing
legacy db entries so that they may be updated to the
new json format for this role field.

Note that as previous db entries may have non json
format we account for this as before and test for
json format before querying mdraid property existence
in role field.

As the role field was previously used only as an mdraid member
flag mechanism it would wipe all other roles, this was improved
in the case mdriad members having additional roles but all roles
would still be lost on non mdraid members. This is the initial
patch to address roles retention in non mdraid members.

As the default db entry and the cheapest value to check is null, make
sure we return to null when we remove the 'mdraid' member role and
it is the last role. Also avoids unnecessary conversion to json and
quicker reads for the majority case (data disks).