admin: Migrate from Highcharts to C3

For every chart that uses Highcharts, add its C3 counterpart below it.
The change from Highcharts to C3 is for license reasons.
admin: Lift restriction on charts

Remove `ADMIN_ENABLE_CHARTS`, as we now use C3 instead of Highcharts.
docs: Remove Admin charts section

Due to the migration from Highcharts to C3, the charts are now enabled
by default.
Revert "cyclades: Invalidate snapshots when job fails"

This reverts commit 0d1b405df358e17e369d910edc0fa9977b12bfa9 that has
been accidentally pushed remotely.
Merge branch 'release-0.16-admin-charts' of https://github.com/apyrgio/synnefo into release-0.16
astakos: Fix license of astakos/admin/users.py
astakos: Fix pep8/pyflakes issues
docs: Add a warning for RC versions of CentOS

Currently, we don't provide packages for release candidate versions
of Synnefo for CentOS 6.5. This commit adds a warning in the CentOS
installation guide to reflect this and direct users to an archived
version of the Synnefo 0.15.2 docs.