[DPE-2992] Part 1 - Provide Mongos with keyfile

[MiaAltieri]

Issue

DPE-2992 is to start the mongos subordinate charm with the provided config server. In order for the config server and for mongos to be connected they both must have the same keyfile

Solution

Provide that key file

Follow up PRs

1. A follow up PR on mongos charm that uses this library
2. Updating the library to actually start the mongos service with the information of the config server
3. Integration tests

Testing

juju deploy ./*charm --config role="config-server" config-server
juju deploy application
juju deploy mongos
juju integrate application mongos
juju integrate mongos:cluster mongodb:cluster
juju ssh mongos
sudo cat KEY_FILE_PATH