Add ability to configure retry parameter for apache proxy

Retry parameter is useful to define maintenance window during
which backend will be unavailable in case it is in error state.
During this period of time connections to the backend won't be
allowed, and apache will give 503 error. More details: [1]

[1] http://httpd.apache.org/docs/2.2/mod/mod_proxy.html#proxypass

Change-Id: I324fc9c7ecc5f7a68b9c04946693f383453013d4
Related-Prod: https://mirantis.jira.com/browse/PROD-25993
4 files changed