plainbox:applogic: add extcmd option to PlainBoxConfig

This patch extends the base PlainBoxConfig class to include a new
section (FEATURE-FLAGS) with a new variable (extcmd). The intent of this
(and others later) feature flag is to allow users to opt-into using new
features without having them broadly enabled just yet.

To *enable* and use the new glibc-based extcmd, please add the following
section to ~/.config/plainbox.conf (the default is "legacy")

[FEATURE-FLAGS]
extcmd = glibc

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
plainbox:runner: respect the extcmd feature flag

This patch connects the JobRunner to the new feature fag that controls
glibc-based extcmd implementation. As before, this only applies to
systems running Python 3.4+

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
"automatic merge of lp:~zkrynicki/checkbox/plainbox-opt-in-flags/ by tarmac [r=sylvain-pineau][bug=][author=zkrynicki]"