mock and systemd nspawn

[pcfe]

note to self:
config_opts['use_nspawn'] = False
to

• docs
• example config el7
• example config el6
  if in testing I continue having issues with mock builds with the latest mock

[pcfe]

note to self: at $current_customer disabling nspawn made mock builds for el6 and el7 work again.
Debug in the w-e on home setup (where it's more acceptable to have a couple days of broken builds)

[pcfe]

note to self:
yeah systemd-nspawn enabled builds also broken on home-setup (that's a RHEL7 box building RHEL7 packages)

INFO: mock.py version 1.4.2 starting (python version = 2.7.5)...
Start: init plugins
INFO: selinux disabled
Finish: init plugins
Start: run
INFO: Start(./bats.spec)  Config(rhel-7-x86_64)
Start: clean chroot
Finish: clean chroot
Start: chroot init
INFO: calling preinit hooks
INFO: enabled root cache
Start: unpacking root cache
Finish: unpacking root cache
INFO: enabled yum cache
Start: cleaning yum metadata
Finish: cleaning yum metadata
INFO: enabled HW Info plugin
Mock Version: 1.4.2
INFO: Mock Version: 1.4.2
ERROR: Exception(./bats.spec) Config(rhel-7-x86_64) 1 minutes 31 seconds
INFO: Results and/or logs in: /var/lib/jenkins/workspace/after-github-pr/tmp/srpms
INFO: Cleaning up build root ('cleanup_on_failure=True')
Start: clean chroot
Finish: clean chroot
ERROR: Command failed: 
 # /usr/bin/systemd-nspawn -q -M b020e25c9ada4fb5b4ecc9f818b39d3d -D /var/lib/mock/rhel-7-x86_64/root --setenv=LANG=en_US.UTF-8 --setenv=TERM=vt100 --setenv=SHELL=/bin/bash --setenv=HOSTNAME=mock --setenv=PROMPT_COMMAND=printf "\033]0;<mock-chroot>\007" --setenv=HOME=/builddir --setenv=PATH=/usr/bin:/bin:/usr/sbin:/sbin --setenv=PS1=<mock-chroot> \s-\v\$  /usr/sbin/groupadd -g 135 mockbuild

ERROR:   Could not build SRPM bats using the specfile ./bats.spec [Wed Jul 19 17:49:28 CEST 2017]

[pcfe]

PR #75 fixes the problem (simply by disabling the new systemd-nspawn for the mock builds)