Hide default_cgsnapshot_type from cinder group types

The generic group type named "default_cgsnapshot_type" is reserved
for the consistency group and we cannot use a generic group using
"default_cgsnapshot_type". "default_cgsnapshot_type" should not be
listed in "Group Type" selection in "Create Group" form.

In addition, "Group" and "Group Snapshot" panels make no sense
when there is no group type other than "default_cgsnapshot_type".
This commit adds a suggestion to hide "Group" and "Group Snapshot"
panels if there is no valid group type to the release notes.

Closes-Bug: #1785178
Change-Id: I2dce39bcbcf6bedc8dc0c94d11f3c0a85ea61490