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
deploy: Remove version dependency to curl
admin: Fix typos in admin tests
admin: Fix typo in deleted entries tooltip

Remove the "None." string that appeared on every category (besides
projects) due to a bad template tag. Also, add the tooltip in a
separate html file for easier editing.

Fixes apyrgio/synnefo#288
astakosclient: Create exception 'ConnectionError'

When astakosclient fails to connect to the remote server, raise a
'ConnectionError'. This exception contains the original Exception
thrown by httplib so that it can be used by other clients such as kamaki.
admin: Make Admin base URL configurable

Add an `ADMIN_BASE_URL` setting, which works roughly the same as the
Cyclades/Astakos base URLs (minus the service registration part). The
main benefit of the `ADMIN_BASE_URL` setting is that it allows the
administrator to define the path for Admin (or lack thereof).

Also, using the `extends_path_with_slash` function, the base URL that
the administrator chooses should work with or without a slash. In the
latter case, a Django-specific redirect view will be used to redirect
the request to the URL with slash.

Finally, the addition of the `ADMIN_BASE_URL` brings also another
change. If the administrator wants to install Admin in a separate node,
he/she no longer has to choose between using as ROOT_URLCONF the
'synnefo_admin.urls' or 'synnefo_admin.admin.urls' (their main
difference was that the former had '/admin/' hard-coded).
Using simply the 'synnefo_admin.urls' should suffice,
since the `BASE_PATH` will be extracted from the `ADMIN_BASE_URL`.

Fixes apyrgio/synnefo#289
admin: Fix selection of default catalog view

Return the highest-order view type in the `ADMIN_VIEWS_ORDER` setting.
admin: Add URL test for Admin

Add a test that checks if the `ADMIN_BASE_URL` setting is used by Admin
and if we have any issues with slashes and redirections.
admin: Add tests for default catalog view
docs: Update install/admin guides

Update the install/admin guides to reflect the addition of the
`ADMIN_BASE_URL` setting. Also, add an Admin section in the CentOS
install guide.
deploy: Add `ADMIN_BASE_URL` in Admin conf

Add an `ADMIN_BASE_URL` setting in the admin.conf file. The value of
this setting is created in the same fashion as the values for
CYCLADES_BASE_URL / ASTAKOS_BASE_URL.
Mention the `ADMIN_BASE_URL` setting in Changelog
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.
astakos: Remove unused file admin/users.py