mgr/dashboard: fix pool usage calculation

mgr/dashboard: fix pool usage calculation

Currently Dashboard Pool usage calculation does not match the output of
'ceph df' command.

Fixes: https://tracker.ceph.com/issues/45185
Signed-off-by: Ernesto Puerta <epuertat@redhat.com>
(cherry picked from commit b4a9dc17a3de90379964443d26b29f1759824f28)

Conflicts:
	qa/tasks/mgr/dashboard/helper.py
	src/mon/PGMap.cc
	src/pybind/mgr/dashboard/frontend/src/app/ceph/pool/pool-list/pool-list.component.html
	src/pybind/mgr/dashboard/frontend/src/app/ceph/pool/pool-list/pool-list.component.spec.ts
	src/pybind/mgr/dashboard/frontend/src/app/ceph/pool/pool-list/pool-list.component.ts
	src/pybind/mgr/dashboard/frontend/src/app/shared/components/usage-bar/usage-bar.component.ts:
        - Keep UsageBar component totalBytes/usedBytes names
        - Bring new UsageBar option decimal
Signed-off-by: Ernesto Puerta <epuertat@redhat.com>