Add config option to change default number of admin and public process weights

[Marcin Wilk]

Currently, an administrator can modify a number of the keystone wsgi app processes by changing the 'worker-multiplier' config option. However the actual number of the wsgi processes for the admin and public endpoints, which is derived from the aforementioned config option, always remains in the fixed ratio of 1/3 (0.25/0.75) [1].
There are scenarios where admin endpoint is heavily used comparing to the public endpoint and it would be nice to expose those weights, in some form, as charm config option(s). Ie, single option admin/public factor (float). Value of 1 would mean the same number of processes for both endpoints, 2 - twice as much for admin vs public, 0.5 - twice as much for public vs admin, and so on.

Currently, as these numbers are bound to each other, an administrator can't increase/decrease one without increasing/decreasing the other. For example if 30 admin processes are needed, 90 public will be configured as a consequence of changing the 'worker-multiplier', even though that so many public processes are not needed (they will use cpu, mem resources and db connections).

[1] https://opendev.org/openstack/charm-keystone/src/branch/master/charmhelpers/contrib/openstack/context.py#L1739

[Alex Kavanagh (ajkavanagh)]

This could be added as a config option on the charm, and then accessed in charm-helpers in the context. Please discuss the implementation here (in this bug) before implementation.