bloodhound plugin compatiblity with neo4j 4

[svenhertle]

The bloodhound plugin did not work with neo4j version 4. This pull requests fixes:

• Deprecated Cypher statements ({props} vs. $props)
• Creation of existing constraints fails (now, the error is ignored)
• TLS is used, but neo4j does not offer it by default (the encrypt parameter is set to False now)

The last thing feels a bit wrong, but as far as I see Bloodhound does it the same way for now. Bloodhound-AzureAD also has some problems with neo4j version 4, maybe I fix them later ;)

I tested this with neo4j 4.0.5 and 3.5.17 now.

Thanks for the awesome tool!

[dirkjanm]

thanks for your contribution! just merged it in to master