(PUP-1932) Restrict systemd provider to manage file-based services

Prior to this commit, the systemd provider returned in-memory services.
This had a couple odd consequences, e.g. with 'puppet resource service':
1) short-lived services would come and go, though they are not actually
   managed directly
2) disabled and stopped services wouldn't be reported at all

This commit changes the systemd provider to return file-backed
services.
Use no-daemonize and not forking service

Making puppet fork and systemd to read PID file seems unnecessary complicated.
Adding dependency on puppetmaster to puppet

Sounds like a good idea to wait with start of puppet agent to finish starting
of puppet mater first.
Make relation between service and sysconfig file obvious

There is puppet service but it's environment file is called puppetagent
(probably due to historical reasons). Making the naming more consistent and
obvious.
Merge pull request #2507 from kylog/issue/pup-1932/systemd-reports-transient-services

(PUP-1932) Restrict systemd provider to manage file-based services
Reading even old sysconfig file

For compatibility reasons reading even the old sysconfig file although new one
overrides it. But people who have some interesting settings in the old
sysconfig file can still use them unless the are set to something else in the
new one.
Merge pull request #2510 from miska/master

Little bit of systemd unit files polishing
Add the constraint type and check constraints before running a transaction.

Constraints allow modules to express dependencies on resources
that are managed by other parts of the manifest, most likely
other modules.

They are an alternative to the ensure_resource function from stdlib
as well as the common workaround of declaring shared resources inside
`if !defined(Ref[resource])` blocks.

The idea and initial design for constraints were originally shared by
John Bollinger on the mailing list:
https://groups.google.com/d/msg/puppet-users/Fvl0aOe4RPE/Ph38bq3FmHcJ
TEMP: describe constraints in README.md