plainbox:result: add _JobResultBase.append_comments()

This patch adds a new method to the base result class, shared by both
MemoryJobResult and DiskJobResult, which simplifies appending comments
regardless of the current value of the comments field. Since comments
can be (and are by default) None, this method simply hides the
commonly-used if-then-else pattern that was common in several places
that handled comments.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
"automatic merge by tarmac [r=sylvain-pineau][bug=][author=zkrynicki]"