OVSvApp monitoring changes to support fault tolerance

With the introduction of OVSvApp solution, which consists of
a service VM hosted on ESX hosts within a data center.

To address the Fault Tolerance and High Availablity of OVSvApp.

OVSvApp solution caters to the following failure scenarios:

OVSvApp agent crash:
  - upstart mechanism tries to bring up the agent back.

Open vSwitch process crash:
  - Child process of OVSvApp agent will monitor the OpenvSwitch
    and tries to restart it when crashed.
  - This will help us to re-populate the flows at the earliest
     to resume the tenant VM data network connectivity.

OVSvApp VM crash:
  - Agent monitoring mechanism will put ESX host in maintenance mode
    or powers off as per "esx_maintenance_mode" param configuration.
    By default, 'esx_maintenance_mode' is True, so we put the host to
    maintenance mode.
  - vCenter DRS will migrate tenant VMs to other ESX hosts within the
    cluster and safeguards traffic from being blackholed.
  - Agent monitoring can be turned on/off using the configuration
    "enable_agent_monitor" flag configuration in neutron configuration.
    By default, it is enabled.