Allow connection string to be just backend name

Having to specify backend connection as URI is somewhat confusing
when other parts of uri (like host or path) are not used. This commit
allows to specify just backend name as connection string.

Partially implements blueprint persistence-uris

Change-Id: Ic38d41d4f24dcd596cbdff33de78d1a137fb2e8f
Merge "Allow connection string to be just backend name"