Allow datastore key to be populated from a variable.

[nmaludy]

Previously, if installing StackStorm in a multi-node environment, it would generate a new datastore key on every node.

This change allows the datastore key to be specified as a variable st2_datastore_key then propagated to each host.

By default, this variable is null and will result in a random key being generated (retaining same behavior).

[CLAassistant]

All committers have signed the CLA.