fleetd: process dependencies in [Install] section

[dongsupark]

Introduce a section [Install] in unit files, to allow WantedBy in the section to be able to enable the dependent units.
This PR is just a rebased version of #1523, except for minor changes to fix build errors.

Resolves #1382
Supersedes #1523

/cc @kayrus

[dongsupark]

Hmm, interesting. TestInstallUnit fails only on semaphoreci, hanging forever on "fleetctl load". OTOH local test runs without any problem.
Do not merge this PR until the test issue could get fixed.

[kayrus]

@dongsupark last one doesn't relate to install stuff.

[dongsupark]

@kayrus Right. I was just taking a try-and-error approach. But none of the reverted patches was a culprit.
Though I think I just figured out how to reproduce the hanging problem on my local laptop, testing with CoreOS alpha.
I'll let you know if I find more about this.

[dongsupark]

To sum up, this PR works fine on CoreOS alpha v1151.0.0 (with systemd v231), while it still fails with CoreOS beta(1122) & stable(1068) (with systemd v229). So the issue of failing functional test was clearly not a fleet issue.
Once CoreOS stable got updated to 1151, I would merge this PR.

[dongsupark]

CoreOS stable was updated to 1185.3.0 with systemd v231. So finally this PR can be merged.

However it's still pending due to other test failures. It's blocked by #1700.

[dongsupark]

All right. As the blocker #1700 was merged, I'll also merge this PR.

[jonboulle]

Bit late to this. Code LGTM, but @dongsupark what do you think about making this configurable, since it can seem a bit unintuitive?

[dongsupark]

@jonboulle I agree that it's unintuitive, but I'm not sure about how to make it configurable for users. AFAICS users anyway need to write own unit files for dependencies, with either Wants or WantedBy. Can you please explain?