[rpm] Add Signature tag to the output of 'installed-rpms'

 RPM meta data consists of Signature,Packager,BuildHost,Vendor tags
 it is useful for identifying 3rd party packages present on a system.
 This patch adds the signature column to installed-rpms.

Signed-off-by: Poornima M. Kshirsagar <pkshiras@redhat.com>