Nova disables vTPM support by default and users have to set
the [libvirt] swtpm_enabled option to enable it. So the package
installation should be optional.
Note:
The parameter was introduced during this cycle[1] so we can safely
change the default value.
[1] e8a14766c8
Change-Id: I2e1791dcc64a844691a380686e695efeaaa0e343