Description of problem:
In the node code - lib/openshift-origin-node/model/unix_user.rb - the write_ssh_keys method is not thread safe. 


Version-Release number of selected component (if applicable):
all

How reproducible:
Often (on INT)


Steps to Reproduce:
1. Create an account on integration
2. Add 2 different ssh keys to the account
3. In a loop - create and destroy scalable apps till the git clone fails.
  
Actual results:
Fails to clone the git repository because the ~/.ssh/authorized_keys is hosed.


Expected results:
git clone should succeed + keys should be valid. 

Additional info:
The broker creates the gear and then issues parallel mcollective calls to add the authorized ssh keys to the gear. On a slow system, its possible to have the 2 parallel ssh key operations running simultaneously and this results in the 
~/.ssh/authorized_keys file getting borked.