Commit message:
```
Add CentOS packages, unbreak MASTER_SITES and fix nss
- add the following packages:
- libicu
- libunwind
- lttng-ust
- userspace-rcu (aka urcu)
- bump RPM version for:
- pulseaudio-libs (to version 26)
- libgfortrain (to version 18)
- avahi-libs (to version 17)
- nss:
- indicate that the versions of nss we ship have been patched [1][2]
- add nss-pem package for CentOS 7, for CentOS 6 it was included in the nss package
- EPEL:
- upstream changed the layout of their mirrors
- packages are under the directories Packages/[0-9a-z]
- EPEL7 already included the latter prefix ([0-9a-z]) in SUBDIR
[1] https://lists.centos.org/pipermail/centos-announce/2017-September/022552.html
[2] https://lists.centos.org/pipermail/centos-announce/2017-September/022550.html
```