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.

We also switch to "sh" instead of %x on the call to ensure failures raise
exceptions.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
Merge pull request #219 from MosesMendoza/maint/master/promotion_fixes

Signing changes for promotion workflow