Description of problem:
rbd mirror snapshot schedule add doesn't support image-spec as user input.
It would be nice if we can support the image spec as an input to maintain uniformity in command input support across the commands

Version-Release number of selected component (if applicable):
16.2.7-40.el8cp

How reproducible:
Always

Steps to Reproduce:
1. Configure mirroring - snapshot based mirroring
2. Add snapshot schedule and try to provide image-spec as an input similar to most of the commands


Actual results:
# rbd mirror snapshot schedule add two_way_rep_snapshot_image/for_2020618 1m
rbd: rbd mirror snapshot schedule add failed: (22) Invalid argument: Invalid interval (two_way_rep_snapshot_image/for_2020618)


Expected results:
Support for image-spec as a valid user input for pool name and image name

Additional info:
# rbd help mirror image status two_way_rep_snapshot_image/big                             
usage: rbd mirror image status [--pool <pool>] [--namespace <namespace>] 
                               [--image <image>] [--format <format>] 
                               [--pretty-format] 
                               <image-spec> 

# rbd help mirror snapshot schedule add two_way_rep_snapshot_image/for_2020618 1
usage: rbd mirror snapshot schedule add
                                      [--pool <pool>] 
                                      [--namespace <namespace>] 
                                      [--image <image>] 
                                      <interval> <start-time>