1146294 - do not require pulp.bindings.server to access DEFAULT_CA_PATH

The Katello agent uses DEFAULT_CA_PATH, but does not install the Pulp bindings
by default. The constant should live in pulp.common.constants instead.
Merge pull request #1296 from beav/move-constant

1146294 - do not require pulp.bindings.server to access DEFAULT_CA_PATH