Bug 1912 Update GPG Signing

Update the CLI arguments for GPG signing so that it reads the passphrase
passed in via the CLI.  This is needed as the version of GPG has been updated
and no longer accepts the old CLI arguments.

Fixes bug 1912

Change-Id: I0a850b3f1641edb917624be8fd94be78667d01fd
Signed-off-by: beierlm <mark.beierl@canonical.com>