Agent: Fix debian package upgrade issue

[shishirb-MSFT]

• During an upgrade, postrm script deletes the 'do' group causing 'adu' membership to be removed.
• postrm script is now updated to perform cleanup only on remove and purge actions. postinst script already handled the case of existing user/group entities.
• Also move systemctl reset-failed to postinst as the more appropriate location. If retaining in prerm, we will have to take a call on whether to call it only on upgrade/abort-upgrade/etc.

[jw-msft]

unless I'm missing something, it seems like postrm.in.sh and prerm.in.sh need set -e so it will return failure ret code when individual commands fail so that it does not leave the install/upgrade in a partial state.

[shishirb-MSFT]

Agree about set -e but it is not in scope for this PR. Adding set -e requires more changes like deleting file/directory only if it exists.