Switch package install to Python 3 for OpenStack Rocky or later.
When upgrading, remove any python-* packages that were explicitly
installated and then autoremove --purge any dependencies that are
no longer required.
Also drop the python2 shebang from hooks/manager.py in favor of
specifying the interpreter on the subprocess call. The python
interpreter version must match the python version of the OpenStack
payload due to the keystoneclient library imports.
Depends-On: I18996e15d2d08b1dacf0533132eae880cbb9aa32
Change-Id: If973ebc2be3b32ee3ff2122b5874dad96cda9fec
9cb5636eba