replication: fix rebootstrap in case the instance is listed in box.cfg.replication

When checking wheter rejoin is needed, replica loops through all the
instances in box.cfg.replication, which makes it believe that there is a
master holding files, needed by it, since it accounts itself just like
all other instances.
So make replica skip itself when finding an instance which holds files
needed by it, and determining whether rebootstrap is needed.

We already have a working test for the issue, it missed the issue due to
replica.lua replication settings. Fix replica.lua to optionally include
itself in box.cfg.replication, so that the corresponding test works
correctly.

Closes #4759