Enable heat/puppet to manage the fernet keys and make it configurable

Enable heat/puppet to manage the fernet keys and make it configurable

With the addition of the KeystoneFernetKeys parameter, it's now possible
to do fernet key rotations using mistral, by modifying the
KeystoneFernetKeys variable in mistral; subsequently a rotation could
happen when doing a stack update.

So this re-enables the managing of the key files by puppet. However,
this is left configurable, as folks might want to manage those files
out-of-band.

bp keystone-fernet-rotation
Change-Id: Ic82fb8b8a76481a6e588047acf33a036cf444d7d