CA-113392: Fix PBD password-secret lifecycle

When creating an SR, if there are any secrets in the device-config then they
are duplicated and a unique one is used for each PBD creation.

However, XenCenter's "Repair SR" feature creates new PBDs directly, copying the
device-config field from existing ones. This bypasses the secret-duplication
which is in the SR-create code path.

This patch pushes the secret-duplication logic down to the PBD creation which
solves the XenCenter issue pending a revised design of the secret datamodel.
Also the master is no-longer special cased for the same reason. This has the
added benefit of making secret creation/destruction entirely symmetrical: the
secrets created with PBDs are destroyed when the PBD is destroyed and the
secrets created manually must be destroyed manually.

XenCenter should now be updated to tidy up after itself: if it chooses to
create secrets around passwords used in SR creation, then they should be
destroyed in a finally block after the SR creation is complete. This is safe
since all PBDs will have duplicated the secret.

Note this patch applies (as the previous semantics did) to any key in device
config ending in '_secret'. The ones used by the SM backends are
'password_secret' and 'chappassword_secret'.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Merge pull request #1487 from simonjbeaumont/ca-113392

CA-113392: Fix PBD password-secret lifecycle