octavia check is enabled dependent upon charm config instead of endpoint existence

[Drew Freiberger]

Most checks in openstack-service-checks are only implemented if the endpoint is advertised by keystone. However, the octavia check is enabled by charm config by default without regard for octavia being installed or it being a version of openstack that contains octavia.

This should A) not be checked on pre-octavia versions of clouds, even if = true, and
B) should be checking the endpoint catalog for presence of octavia instead of charm config.

$ openstack endpoint list|grep octavia
| 0f8e657635d34a688f61b0b1817f8749 | RegionOne | octavia | load-balancer | True | internal | https://10.0.0.10:9876 |
| eb2ab4e7396a4eb99baf4183a537cdb9 | RegionOne | octavia | load-balancer | True | public | https://10.0.0.10:9876 |
| fa2ddaea6e6b42cab356fc9d1c129679 | RegionOne | octavia | load-balancer | True | admin | https://10.0.0.10:9876 |