Add lock for writing and reading self.runbook.admin_private_key_file

[lubaihua33]

When cocurrency is not 1, and admin_private_key_file in the runbook is empty, there might be VM connection issue.

When multiple LISA threads run concurrently, they contend for access to self.runbook.admin_private_key_file. This leads to a race condition during read/write operations. Specifically, if one thread reads admin_private_key_file as empty and begins generating a new key, another thread may do the same before the first thread finishes writing the file path. As a result, multiple keys are generated simultaneously. The VM creation process may use one key, while the connection attempt uses another, causing the VM to become unreachable.

This PR adds a lock for self.runbook.admin_private_key_file