Adds ability to configure various clients used by the Heat

This commit adds config sections [clients_nova], [clients_swift],
[clients_neutron], [clients_cinder], [clients_ceilometer] and
[clients_keystone]. These sections contain additional configuration
options for corresponding OpenStack clients.
Currently those are only SSL-related setting ca_file, cert_file,
key_file and insecure. Note, than not every client library is
currently capable of utilizing all of the SSL settings.

There is also a plain [clients] section that holds shared client
options. Each option searched first at specific group (clients_xxx)
and if it not found there then the value from [clients] group
are taken (or default values if there is no such setting in this
group). This allows defining shared configuration that would be
used by most (or all) clients without repeating the same settings
for each and every client separately

Closes-Bug: #1213122
Implements: blueprint clients-ssl-options

Change-Id: Id9ccbffce0d5c266202fdb1cf24a9ffb63e507e6
Merge "Adds ability to configure various clients used by the Heat"