absolute paths for conffile in debian pkg

[legleux]

High Level Overview of Change

Install of deb packages will fail on new Debian 11 Bullseye with error:
dpkg: error processing archive build/dpkg/packages/rippled_1.8.0~b4-1_amd64.deb (--install): conffile name 'opt/ripple/etc/rippled.cfg' is not an absolute pathname

Context of Change

Absolute paths weren't enforced in deb packages before.
https://git.dpkg.org/cgit/dpkg/dpkg.git/commit/?id=15416a18f7645dbb9a36edb1e52f30a9e83c6627

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (non-breaking change that only restructures code)
• Tests (You added tests for code that already exists, or your new feature included in this PR)
• Documentation Updates
• Release

[ximinez]

I noticed one of the Travis builds failed, and I cleared the cache and am re-running the dependency steps before I try that one. However, since this change doesn't affect the build at all, that's no reason to hold this up.

[scottschurr]

@manojsdoshi, do you have any comments on this pull request? With your approval we could mark it as passed. Thanks.

[manojsdoshi]

LGTM