Add support for "No Proxy Host" list

The Jenkins proxy configuration supports a whitelist of nodes that will
not use the the proxy when Jenkins (or some plugins) attempts to
communicate. As of right now, this setting will be cleared by Puppet
if any other aspect of the proxy configuration is managed.

This commit adds support for the "No Proxy Whitelist" by adding a new
parameter to the `jenkins` class. The new parameter, `no_proxy_list`,
will accept an array of hostname patterns to no longer use the proxy to
access. The `no_proxy_list` parameter is optional and will default to
`undef`.
Add tests for the `no_proxy_list` parameter

This commit adds tests to verify the behaviour of the `no_proxy_list`
parameter on the `::jenkins` class. The tests verify what happens when
the parameter is set and unset.
Improve how `::jenkins::proxy` accesses variables

Without this commit, the template referenced by the class
`::jenkins::proxy` was using the `lookupvar()` method in the ERB
tags to access variables from other class scopes. This method of
accessing the variables is at the mercy of changes to Puppet's internal
Ruby API. This method is also not very clear as to what is happening
without having to read through both the class and the template.

This commit changes the method of variable access to use a simple
variable assignment operation in the Puppet DSL to bring the values of
the required variables into the local scope. This is preferred as it
makes the template easier to read and keeps the template's operations
relative to the place from where it is called. In the Puppet code
itself, it is now more clear as to what data is used by the
`::jenkins::proxy` class and where that data is coming from.
Add documentation for `no_proxy_list` parameter
Merge pull request #202 from trlinkin/proxy_work

Proxy work