Added a script for building and deploying Debian packages to launchpad.net

Summary:
Up to now, we've been relying on sickpig to manually maintain the Bitcoin ABC Ubuntu PPA.
This script gets us closer to automating that process, allowing one click to generate packages for Xenial (16.04), Bionic (18.04), and Eoan (19.10, latest release)
and upload them to the PPA.

As demonstration, I used this script to upload 0.21.2.9 and 0.21.2.10 to a personal PPA: https://launchpad.net/~jasonbcox/+archive/ubuntu/bitcoin-abc
(the .9 and .10 versions are patch versions, which are not expected to be used in production except for hotfix type situations)

Test Plan:

./debian-packages.sh -h
./debian-packages.sh --dry-run 0.21.2.9 "Jason B. Cox <jasonbcox@bitcoinabc.org>"

cat >> ~/.dput.cf <<EOF

[jasonbcox-bitcoin-abc]
fqdn = ppa.launchpad.net
method = ftp
incoming = ~jasonbcox/ubuntu/bitcoin-abc/
login = anonymous
allow_unsigned_uploads = 0
EOF
./debian-packages.sh --ppa jasonbcox-bitcoin-abc 0.21.2.9 "Jason B. Cox <jasonbcox@bitcoinabc.org>"

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5472