Anaconda: temporarily set default version to 2019.07

Bug: ContinuumIO/anaconda-issues#11466
Latest version 2019.10 is affected by a bug that prevents a successful installation on virtual machines with 1 vcpu.
Temporarily set the default Anacoda installer version to 2019.07 waiting for the fixed Anaconda release.

elasticluster/share/playbooks/roles/anaconda/defaults/main.yml

@@ -7,7 +7,7 @@
 anaconda_mirror : 'http://repo.continuum.io/archive'
 
 # Version of the Anaconda distribution to install
-anaconda_version : '2019.10'
+anaconda_version : '2019.07'
 
 # Anaconda comes with either a Python2 or a Python3 interpreter
 # -- choose which one you want here.