Customizing deploy key and ssh wrapper path for git cloning through application_git cookbook
Quoting from the cookbook README:


  deploy_key – SSH key to use with git. Can be specified either as a
  path to key file already created or as a string value containing the
  key directly.


And:


  The application_git resource deploys code from git. It extends the
  core git resource to support deploy keys and disabling strict host key
  verification.


So checking the core git resource documentation:


  ssh_wrapper Ruby Type: String
  
  The path to the wrapper script used when running SSH with git. The
  GIT_SSH environment variable is set to this.


so something like this (untested) should do (using the wrapper in the cookbook) :

cookbook_file '/path/to/my/desired/ssh_wrapper.sh' do
  source 'my_ssh_wrapper.sh'
end

application 'accounts' do
  action :deploy
  path &lt;somepath&gt;
  owner 'nobody'
  group 'nogroup'
  git 'name' do
    repository &lt;repopath&gt;
    revision &lt;value&gt;
    ssh_wrapper '/path/to/my/desired/ssh_wrapper.sh'
    deploy_key deploy_key
 end
end 


You can of course do the same with the deploy_key, but using something like chef_vault or an encrypted databag sounds better to avoid commiting deploy keys within the cookbook.