xci: get-opnfv-scenario-requirements.yml: Check if facts are set

xci: get-opnfv-scenario-requirements.yml: Check if facts are set

When the user selects an invalid combination then some facts may not be
set so the deployment may fail in a crypt way like in the following case

fatal: [localhost]: FAILED! => {"failed": true, "msg": "The conditional
check 'deploy_scenario_installer' failed. The error was: error while
evaluating conditional (deploy_scenario_installer):
'deploy_scenario_installer' is undefined\n\nThe error appears to have been in
'/home/opnfv/releng-xci/xci/playbooks/get-opnfv-scenario-requirements.yml':
line 114, column 11, but may\nbe elsewhere in the file depending on the
exact syntax problem.\n\nThe offending line appears to be:\n\n
when: item.installer == XCI_INSTALLER\n        - set_fact:\n
          ^
here\n"}

Change-Id: I931376d99bb178b15c4a9a71d47b48b01b858e4e
Signed-off-by: Markos Chandras <mchandras@suse.de>