Detect absence of 'use_ipv6' stanza in /etc/nova/nova.conf

- 'use_ipv6' configuration stanza is commented by default in
  /etc/nova/nova.conf.  We need to check return code of grep invocation
  to detect this.  If stanza is commented we consider IPv4.

Closes-bug: #1349432
Change-Id: I6311d02e4bb0ab9e62e5d82c12648cc509d67a9b