Allow noarch packages to be found for yum installation.

I'm attempting to install a number of "noarch" packages via a Salt state, such as Python-based packages via EPEL. Because their architecture doesn't exactly match my minon's architecture (eg 'noarch' vs 'x86_64'), the yumpkg ``check_db`` function determines that nothing can be found.  This pull request modifies the architecture check, as 'noarch' packages are installable on any architecture type. 

My Salt states are like this:

```
supervisor:
    pkg.installed
```
with the underlying package named something like ``supervisor-2.1-3.el6.noarch``.  The ``yumpkg`` module check fails to find this package (at https://github.com/saltstack/salt/blob/develop/salt/modules/yumpkg.py#L396) because the conditional statement only checks for an exact architecture match.

Separately, I discovered from reading the code I could use ``supervisor.noarch`` as the package name, but the call to ``yum.searchPackages`` (https://github.com/saltstack/salt/blob/develop/salt/modules/yumpkg.py#L395) returns nothing  using this as the full package name.  Seemingly the format of ``packagename.architecture`` isn't supported here as nothing is returned by the underlying yum module.  

This change appears to be the most sensible to solve this issue.