support for 'user' argument to su to another user when executing git and R10K

[GeoffWilliams]

I've taken on-board the suggestions from https://github.com/acidprime/r10k/issues/135 and developed this PR to address it.

The patch works by checking for the presence of a user argument. If this is present and matches the regexp ^\w+$, we will try to invoke git or r10k by doing su - $user -c 'COMMAND'.

I've modified the agent to support user on all operations and verified that I still receive output and errors, as you can see from the following console captures:

r10k status

peadmin@xmaster:~$ mco rpc  r10k status user=r10k path=/etc/puppetlabs/puppet/environments/production/modules/ntp -v
Discovering hosts using the mc method for 2 second(s) .... 1

 * [ =========================================================> ] 1 / 1


xmaster.vagrant.vm                      : OK
    {:path=>"/etc/puppetlabs/puppet/environments/production/modules/ntp",     :output=>      "# Not currently on any branch.\nnothing to commit (working directory clean)",     :error=>"",     :status=>0}

r10k deploy_module

peadmin@xmaster:~$ mco rpc r10k deploy_module module_name=ntp user=r10k environment=production
Discovering hosts using the mc method for 2 second(s) .... 1

 * [ =========================================================> ] 1 / 1


xmaster.vagrant.vm                       
            Errors:
   Specific module: ntp
            Output:
            status: 0

r10k sync

peadmin@xmaster:~$ mco rpc  r10k sync user=r10k -v
Discovering hosts using the mc method for 2 second(s) .... 1

 * [ =========================================================> ] 1 / 1


xmaster.vagrant.vm                      : OK
    {:output=>"",     :error=>      "[R10K::Action::Deploy::Environment - ERROR] Command exited with non-zero exit code:\nCommand: git --git-dir /etc/puppetlabs/puppet/environments/production/modules/ruby/.git --work-tree /etc/puppetlabs/puppet/environments/production/modules/ruby remote set-url cache /var/cache/r10k/https---github.com-puppetlabs-puppetlabs-ruby.git\nStderr:\nfatal: No such remote 'cache'\nExit code: 128",     :status=>1}

As you can see, there is a strange error here caused by a partial failure and this gets captured too. I got the same error running r10k on the command line so its legit

Thanks Eli, Brett and Zack for your help figuring this one out.

[acidprime]

Looks good @GeoffWilliams can you target https://github.com/acidprime/r10k/tree/3.x as I want this functionality in , but may want to refactor a little.

[GeoffWilliams]

Re-created as https://github.com/acidprime/r10k/pull/142/