(#14297) Handle upstart services better

The previous changes to the upstart provider didn't take into account services
that may have upstart jobs in /etc/init with no corresponding symlink to
upstart-job in /etc/init.d. This fix adds /etc/init/$service.conf to the search
path in the upstart provider.

In order to allow upstart to use debian as its parent, this commit adds methods
for enabled?, enable and disable. Without this fix, using a debian style init
script on ubuntu requires manually specifying the debian provider be used.
With this commit, the upstart provider can be the default for ubuntu and
still fail up to its parent, debian.

The enabled?, disable, and enable methods are complicated because upstart has 3
behaviors to account for. Upstart < 0.6.7 have only the conf file and start on
stanzas to worry about, 0.6.7 < upstart < 0.9.0 has a manual stanza which
removes any previous start on declaration, and upstart >= 0.9.0 has an override
file. In upstart >= 0.9.0, the provider does all of its work using override files
to leave the upstart conf file for the service intact and unchanged.

Because of the need to know which upstart version is being used, this commit
also adds a method and a class variable to hold and access the upstart version,
obtained by a call to initctl --version.