create doveadm rmb check command

[jrse]

To check the current plugin configuration, a rmb check command may be helpful.

The check could:

• evaluate the accessibility of the ceph cluster,
• existence of configured storage pool / and alternative storage pool
• write access to configure mailbox directory.
• access rights to storage pool

[jrse]

common problems:

• file permissions (certificate)
• default user of rmb tool is admin
• insufficient ceph access rights (pool, mds,,...)