MINOR: cli: implement wait on be-removable

MINOR: cli: implement wait on be-removable

Implement be-removable argument to CLI wait. This is implemented via
be_check_for_deletion() invokation, also used by "del backend" handler.

The objective is to test whether a backend instance can be removed. If
this is not the case, the command may returns immediately if the target
proxy is incompatible with dynamic removal or if a user action is
required. Else, the command will wait until the temporary restriction is
lifted.