[apt] Only create dd-agent group if it doesn't already exist

[suhasgaddam]

It looks like parentheses are needed for the appropriate order of operations.

[olivielpeau]

Nice catch @suhasgaddam, thanks! You're right in sh the && and || operators have the same precedence, so the parentheses are needed here.

FYI this change only affects the creation of the dd-agent group, I'll update the PR title accordingly. Also, the addgroup command returns 0 when the group already exists so this mistake in the script didn't have any user impact.

Merging, thanks again!