Pin ansible-core at 2.15.6

We are seeing the following issue since 2.15.7 was released:

    Pattern and root_dir should be of the same type, not <class
    'ansible.utils.unsafe_proxy.AnsibleUnsafeText'> and <class 'str'>

Change-Id: I5cb458132133d171037f8697072e6bd8858db01a

requirements.txt

 pbr>=2.0 # Apache-2.0
 Jinja2>3 # BSD
 ansible>=7,<9.0 # GPLv3
+# NOTE(priteau): Temporary pin while we figure out issues with new ansible-core
+ansible-core<2.15.7 # GPLv3
 cliff>=3.1.0 # Apache
 netaddr!=0.7.16,>=0.7.13 # BSD
 PyYAML>=3.10.0 # MIT