Slow tasks for plugins

[todd-a-jacobs]

The plugin tasks are quite slow, probably because they are running whether or not the plugins are currently at the correct committish. This section:

TASK [zzet.rbenv : install plugins for selected users] *************************
ok: [default] => (item=[u'vagrant', {u'repo': u'https://github.com/rbenv/rbenv-vars.git', u'version': u'v1.2.0', u'name': u'rbenv-vars'}])
ok: [default] => (item=[u'vagrant', {u'repo': u'https://github.com/rbenv/ruby-build.git', u'version': u'master', u'name': u'ruby-build'}])
ok: [default] => (item=[u'vagrant', {u'repo': u'https://github.com/rbenv/rbenv-default-gems.git', u'version': u'ead67889c91c53ad967f85f5a89d986fdb98f6fb', u'name': u'rbenv-default-gems'}])
ok: [default] => (item=[u'vagrant', {u'repo': u'https://github.com/rbenv/rbenv-installer.git', u'version': u'bc21e7055dcc8f5f9bc66ce0c78cc9ae0c28cd7a', u'name': u'rbenv-installer'}])
ok: [default] => (item=[u'vagrant', {u'repo': u'https://github.com/rkh/rbenv-update.git', u'version': u'1961fa180280bb50b64cbbffe6a5df7cf70f5e50', u'name': u'rbenv-update'}])
ok: [default] => (item=[u'vagrant', {u'repo': u'https://github.com/rkh/rbenv-whatis.git', u'version': u'v1.0.0', u'name': u'rbenv-whatis'}])
ok: [default] => (item=[u'vagrant', {u'repo': u'https://github.com/rkh/rbenv-use.git', u'version': u'v1.0.0', u'name': u'rbenv-use'}])`

could potentially be sped up a great deal by simply checking out the correct commitish (if it exists), and only perform a clone, fetch, or pull if the repository doesn't exist or doesn't contain the necessary commits.