Misc fixes to new dynamic jenkins task creation

Various fixes
Only create a job if it doesn't exist
Simplify methods
Update vendored jenkins_api_client to use 0.12.1
Update timestamp utility method to take an argument

The timestamp previously had a hardcoded format. This commit updates the
timestamp to allow an optional separator, which provides at least some minimal
flexibility.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>