Add support to define variables in configuration

Please add support to define variables in the btrbk configuration file in the syntax like
`machine = "server_foo"`
or even using the Perl scalar syntax
`$machine = "server_foo"`

Background is, I'm going to merge btrbk configurations of several machines. The main configuration is almost identical, just target send-receive is different for the machine name.
Merging would be more comfortable to have 1 line defining the variable and having the rest identical.
Additionally it would be interesting to source other configurations (#492).

Start by locating btrbk's configuration parser and reviewing how configuration values are consumed. Define what variable syntax and substitution rules should mean, including whether Perl-style scalars are supported; done means a configuration can define a machine name once and reuse it for the differing target send-receive settings without breaking existing configuration syntax.

Written by the indexing model from the issue text.