mgr/dashboard: Map dev 'releases' to master

mgr/dashboard: Map dev 'releases' to master

In CephReleaseNamePipe, we used to blindly return the "release name" portion of
the version string. This ends up e.g. returning 'nautilus' for master right
now, which causes us to link to nonexistent documentation on ceph.com.  This
change causes builds marked as 'dev' (as opposed to 'stable') to report
'master' as their release name.

Fixes: https://tracker.ceph.com/issues/36416
Signed-off-by: Zack Cerza <zack@redhat.com>