Move keystone.token.default_expire_time to token.provider

Move the default_expire_time function from
keystone.token.core to keystone.token.provider. The function
keystone.token.core.default_expire_time is slated for removal
in the K cycle.

As part of the move to non-persistent tokens, the token_api calls
are being moved (as appropriate) to the token_provider_api in
preperation of token_api being deprecated.

Change-Id: Ic61e0dc13c7aabadbf75ae7aec30b30ce78d937b
bp: non-persistent-tokens
Merge "Move keystone.token.default_expire_time to token.provider"