[RFE] boom: support for multi-volumes snapshots

Description of problem:

Like most other enterprise customers, separate out system filesystems for /, /var, /tmp, /var/log, /var/crash, /var/log/audit, /home, etc. When we take a snapshot before patching or another risky activity, we take snapshots of all of these LVs. It would be great to be able to use boom to add a boot entry for snapshots is a situation like above. It is very easy to add a logical volume tag to a number of snapshot LVs (eg "-add tag rhel7.7_snapshot") like the entire list above. What I had in mind is extending the functionality of boom to be able to specify a logical volume tag as the "-rootlv" option instead of just being able to put a specific root snapshot lv. For example:
1. create all the required snapshot lvs for all system logical volumes using option "--addtag rhel7snap" for each.
2. create boom entry using the tag: boom create --title "RHEL7.7 snapshot - 20200121" --rootlv @rhel7snap

Then when the server is rebooted and you specify the "RHEL 7.7 snapshot - 20200121" entry to boot from, it not only boots from the root lv snapshot, but all the lv snapshots associated with all the system logical volumes.