Add a suitable default for '-b'
Add a suitable default for '-b'

Most OpenStack doc projects contain only one book anyway, thus use the
first book found in ./doc/ as the default.

Avoids an unhelpful exception if -b is not specified.

Change-Id: I103ddbcf2cb0b3781926896ae78a26ed86b74257

openstack/openstack-doc-tools,savinash47/openstack-doc-tools,savinash47/openstack-doc-tools,openstack/openstack-doc-tools,savinash47/openstack-doc-tools