mgr/dashboard: Allow quick bootstrap script to use custom images

- if no image provided default image will be used
- the -i flag was added but its not working. so this commit fixes that.
- exporting the CEPHADM_IMAGE in cpeh_cluster.yaml so that bootstrap script can utilize that
- passing image in cephadm shell

Signed-off-by: Afreen Misbah <afreen@ibm.com>