[BUGFIX] Aliasing the index fails if name is used by actual index

If the index is created before an attempt is made to use an alias which
points to a suffixed alias name, the (re-)indexing of nodes failed when
the aliasing was attempted.

This change will check for any actual index being present and removes it
prior to creating an alias again.
Merge pull request #24 from Flowpack/fix-alias-is-actual-index

[BUGFIX] Aliasing the index fails if name is used by actual index