Conventional pwenc service has node-specific secrets file. There
are times when we need to encrypt contents of clustered cache
or other shared data between nodes. This adds a clustered
pwenc module using secret stored in private directory of
ctdb_shared_volume. The clustercache plugin already was designed
for eventual inclusion of private data. This PR enables that
support and is a step in preparing for clustered local user
info.

* add framework for calling pytest-3

* simplify pytest command generation

Ticket: #26679

(automatic login)