Use a SSH pool to manage SSH connection

Use a SSH pool to hold all SSH clients.It allows 4 SSH clients
at most to connect to the SSH server at the same time.
This patchset also enables every SSH client connect to the other
controller when they failed to connect to the current controller.
For example,failed to A,then to B,or,failed to B,then to A.

Fixes bug: 1162251
Change-Id: I86f7f684639034be97ddf2031e61ac6bf3a196ad
Merge "Use a SSH pool to manage SSH connection"