Working charm with unit tests. Various features:
Change the install ownership for token store
It turns out that the 'user' that creates the token store, is the only one that
can access it, apart from root. As Barbican uses a 'barbican' user for the
barbican-worker process, we need to create the token store with 1777 perms (see
https://github.com/opendnssec/SoftHSMv2/issues/185) and also create the initial
token using the barbican user.
Add an initial README.md: This describes the charm, where to get help
and how to use it.