Use agent when using gpg-agent for rpm signing, no tty

This commit updates the gpg signing for the PE package promotion workflow, to
use the agent when available. Since we're doing this with jenkins, where there
is no tty, we also pass --no-tty
Pass flags to gpp down through debsign to use agent

In the promotion workflow, we need to be able to tell debsign to use gpg but
without the default search for a tty. This is because jenkins doesn't give us a
tty. Debsign accepts an alternate program to use for signing via -p. We take
advantage of this to pass gpg along with our flags to debsign.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
Switch to sh when calling debsign to ensure we raise an exception on failure
test