This switches `ansible_mitogen.transport_config.PlayContextSpec.password()` to
Ansible's plugin option framework. As a result
- The relatively recent `ansible_ssh_password` variable is now respected.
- The SSH connection password can be templated and specified as a play
variable. Task variables will probably also work, but testing was blocked
by #1132.
There is a chance this change will cause a regression in another connection
plugin (e.g. mitogen_docker), but nothing turned up in the test suite.
I intend ot migrate other connection configuration to
`ansible_mitogen.transport_config.PlayContextSpec._connect_option()`, the next
candidate is the remote port.
fixes#1106
Alex Willmer