Introducing `pgenv` script

Script relies on the list of targets, kept in `/etc/pgtab` by default.
Sample list is also provided.
Introducing `pgjob`

This is a wrapper for any job (script) to be executed against
one of the targets, defined via `pgenv` tool. Tool collects all
the output and saves into a log named by the script. Return code
is honoured and notifications of different types are sent out.