Allow blank email address for keystone_user type

Adjust the newvalues() regex to allow for a blank string for the email
parameter to keystone_user resources.

Change-Id: I36b3eb54fcc39f810861f217dcf1b02e58265aef
Closes-Bug: #1319464
Merge "Allow blank email address for keystone_user type"
Merge git://github.com/puppetlabs/puppetlabs-keystone