(MCOP-25) Remove implicit requirement on puppet 3.5.x

When fixing MCOP-23 we introduced an implicit hard requirement on puppet
3.5.x.

This commit removes that requirement by first checking if the new
context methods have been defined in the Puppet module before using
them.
Merge pull request #23 from ploubser/bug/master/mcop-25

(MCOP-25) Remove implicit requirement on puppet 3.5.x