YUM: Add options to enable and disable Yum plugins

Adds two options you can define:
* disabled_plugins: A comma-separated list of plugins to disable
* enabled_plugins: A comma-separated list of plugins to enable

This allows you to run bcfg2 with certain plugins enabled or disabled
when they're not set that way in the yum configuration.  This is
useful because the Bcfg2 YUM plugin is initialized before it can read
in any files that might overwrite yum plugin configuration.
Client/Proxy: use keyword arguments to match the correct order

The XMLRPCTransport __init__ method defines several arguments with default
values. The current call missed the use_datetime argument, so that the
client_timeout will set the use_datetime paramenter and the protocol option
was used as timeout (does not work, because a float is required and raises
an exception).
Merge pull request #218 from AlexanderS/fix-arguments

Client/Proxy: use keyword arguments to match the correct order
Merge pull request #216 from jsbillings/yum-plugin-master

YUM: Add options to enable and disable Yum plugins
Set option types on YUM/RPM list options
removed python 2.6-ism
Fixed py2.6-ism properly
Fixed client tool tests on systems without /bin/true