Runtime or Command line configurable options for openonu

Create a command line or configuration file mechanism for configuring openomci library attributes and behaviors.

Currently openonu and the openomci library can be configured by changing openomci configuration dictionary attributes in the adapter class, directly in the code.

For the openomci library defaults see:
pyvoltha/pyvoltha/adapters/extensions/omci/openomci_agent.py

For whats currently overridden see:
voltha-openonu-adapter/python/adapters/brcm_openomci_onu/brcm_openomci_onu_adapter.py

        # Customize OpenOMCI for Broadcom ONUs
        self.broadcom_omci = deepcopy(OpenOmciAgentDefaults)

        self.broadcom_omci['mib-synchronizer']['audit-delay'] = 0  # disable audits as brcm onu wont upload once provisioned
        self.broadcom_omci['mib-synchronizer']['database'] = MibDbLazyWriteDict
        self.broadcom_omci['alarm-synchronizer']['database'] = MibDbVolatileDict
        self.broadcom_omci['omci-capabilities']['tasks']['get-capabilities'] = BrcmCapabilitiesTask

Some of these attributes would be useful as command line or configuration file arguments specified during runtime:

• mib-synchronizer and alarm-synchronizer MIB storage class choice. (string class name)
• Lazy MIB storage class commit interval (seconds). currently not exposed via OpenOmciAgentDefaults
• Reset on failed resync count (integer). currently not exposed via OpenOmciAgentDefaults
• Write unknown ONU MIB type to etcd (boolean). new feature related to VOL-2882
• Others?

Some of these configurations may initially be more valuable than others. This story can capture new configurable options. The setting of these configurations should be done consistently for all of them, either in a configuration file or command line options, whichever makes more sense.