The log directory for nova has the default_t SELinux context and this
prevents rsyslog from accessing nova's logs. This patch ensures that
the file contexts are set properly for nova's logs.
This change also makes nova's log directory configurable using the
`nova_log_dir` variable.
Closes-Bug: 1748911
Change-Id: Iaac69c5807715f50386624602375c89adeeb48a1