Cinder, through Oslo Config, supports references as values in its
configuration options, such as the default value used in
target_ip_address which is $my_ip.
This patch adds the same functionality to cinderlib, which is convenient
for library users that use the default values reported by the
list_supported_drivers for the options to set the values.
In other words, if for example we don't pass target_ip_address for the
LVM driver configuration, then it correctly uses the value of my_ip
field, but if we get the default value and pass it, the driver will
fail, since the literal '$my_ip' is not usable as an IP address.
Change-Id: I038da3637268c9e43c464e841f8c357b212fbc9f
b6ddbcda43