mgr: add read balancer support inside the balancer module

mgr: add read balancer support inside the balancer module

Read balancing may now be managed automatically via the balancer
manager module. Users may choose between two new modes: ``upmap-read``, which
offers upmap and read optimization simultaneously, or ``read``, which may be used
to only optimize reads. Existing balancer commands have also been added to
contain more information about read balancing.

Run the following commands to test the new automatic behavior:
`ceph balancer on` (on by default)
`ceph balancer mode <read|upmap-read>`
`ceph balancer status`

Run the following commands to test the new supervised behavior:
`ceph balancer off`
`ceph balancer mode <read|upmap-read>`
`ceph balancer eval` | `ceph balancer eval <pool-name>`
`ceph balancer eval-verbose` | `ceph balancer eval-verbose <pool-name>`
`ceph balancer optimize <plan-name>`
`ceph balancer show <plan-name>`
`ceph balancer eval <plan-name>`
`ceph balancer execute <plan-name>`

In the balancer module, there is also a new "self_test" function which tests
the module's basic functionality. This test can be triggered with the following
commands:
`ceph mgr module enable selftest`
`ceph mgr self-test module balancer`

Related Trello: https://trello.com/c/sWoKctzL/859-add-read-balancer-support-inside-the-balancer-module
Signed-off-by: Laura Flores <lflores@ibm.com>