Zone handling corrected

An issue was brought up, indicating that we had not understood the
documentation of Citycloud's API correctly. Where we had assumed that
a service identifier was simply a unique ID (UUID), it turns out that it
is really a **client-specific** unique ID. Thus, to configure the cloud
adapter, one would first have to figure out which service identifiers
are used for one's particular account.

To make it easier to configure, we have now changed the configuration
file format to allow the user to specify the human-readable zone ID
instead. These identifiers are found in the "Usage" tab under the
"Servers" section of your Citycloud control panel, available at:

    https://citycontrolpanel.com/citycloud#usage

This approach may be bit brittle, since configuration files will become
invalid if Citycloud ever changes the names of these zones, but the
error message generated will output the valid names.