Add rpm-sign to all build hosts

Prior to this commit, rpm-sign only was installed on fedora hosts by this
module. Now that we're switching to EL 7, we should make sure its present on
all hosts.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
Merge pull request #92 from MosesMendoza/maint/master/update_sign

Add rpm-sign to all build hosts