Added ensure => present to group definition

Added ensure => present to ensure_resources(group...) statement, fixing the following error when creating the user account on CentOS 6.5/Puppet 3.4.2:

Error: Could not create user jetty: Execution of '/usr/sbin/useradd -g jetty -m -r jetty' returned 6: useradd: group 'jetty' does not exist

Error: /Stage[main]/Jetty/User[jetty]/ensure: change from absent to present failed: Could not create user jetty: Execution of '/usr/sbin/useradd -g jetty -m -r jetty' returned 6: useradd: group 'jetty' does not exist