Add configurable http_timeout parameter for Cisco N1K

http_timeout is a constant defined in the code. This change makes it
configurable by providing it as an option in the config file.

DocImpact

Conflicts:

        etc/neutron/plugins/cisco/cisco_plugins.ini
        neutron/plugins/cisco/common/config.py

Closes-Bug: #1332290
Change-Id: I429dd9efa43f6a596933359f4627a7761411e621
(cherry picked from commit a8d67485ab498a647b1f50184755c47b18e97e2c)
Merge "Add configurable http_timeout parameter for Cisco N1K" into stable/icehouse