Added check on plugin.supported_extension_aliases

* Adds check to neutron.api.extensions.PluginAwareExtensionManager which raises
an exception for a common class of bugs in plugins: typographical errors in the
`supported_extension_aliases` property. Also, added and updated unit tests.

Plugin changes:

* bigswitch.plugin: Moves api_extensions_path override to plugin's __init__
method, similar to other plugins.

* cisco.n1kv.n1kv_neutron_plugin: Removes "policy_profile_binding" and
"network_profile_binding" as they don't exist in Neutron currently. Extended
override of api_extensions_path to append cisco extensions path to existing
configured path.

* cisco.network_plugin: Renames "Cisco Credential" to "credential". Adds
api_extension_path override to plugin's __init__ method.

* metaplugin.meta_neutron_plugin: Avoids alias of empty string when
cfg.CONF.META.supported_extension_aliases is an empty string.

* midonet.plugin: Fixes regression from 715b16ac.

* nec.nec_plugin: Extended override of api_extensions_path to append
nec extensions path to existing configured path.

* nicira.NeutronPlugin: Extended override of api_extensions_path to append
NXP_EXT_PATH to existing configured path.

Fixes: bug 1225080

Change-Id: Idcaade221d83c611fcbd87b503b2c8377d106962