Configuration value for OS.SshDir is ignored

When configuring Azure/walinuxagent for a custom distribution or VM via `waagent.conf`, an option is provided to specify the path to SSH configuration, defaulting to `/etc/ssh` for many distributions. VMAccess and VMBackup introduce unfortunate side-effects when deployed on such a customized system, since they hard-code the paths as `/etc/ssh` and ignore the configuration values entirely.

## Example scenario

### Expected

- Configuring `OS.SshDir=/etc/openssh` in `waagent.conf` results in VMAccess `reset_ssh` resetting the configuration files within `/etc/openssh`.
- Host key configuration refers to configured `/etc/openssh/ssh_host_*_key` when installing defaults.

### Actual

- Configuring `OS.SshDir=/etc/openssh` in `waagent.conf` results in no change, with VMAccess `reset_ssh` resetting the configuration files within `/etc/ssh`.
- Host key configuration refers to hard-coded `/etc/ssh/ssh_host_*_key` when installing defaults.

## Links

https://github.com/Azure/WALinuxAgent/issues/604