xci: Replace ANSIBLE_VERBOSITY with XCI_ANSIBLE_VERBOSITY

xci: Replace ANSIBLE_VERBOSITY with XCI_ANSIBLE_VERBOSITY

The ANSIBLE_VERBOSITY env variable is actually used internally by
Ansible and exporting it actually makes Ansible super verbose even if
the variable is empty. This feature is going away in the future so we
introduce our own XCI_ANSIBLE_VERBOSITY variable which is simply passed
to the command line to control verbosity in a proper and deterministic
way.

Change-Id: I8a32eeb1a86b4cb0a9872a51bbe9f9624d096e39
Signed-off-by: Markos Chandras <mchandras@suse.de>