only clone docs from gh-pages branch if they aren't there

Allow the caller of this script to decide which commit
from the gh-pages branch should be used to build the docs.
This also avoid excessive repeated git clones during development.

Change-Id: I3e58eef0ac03b15903c06d5632c0eb41413db02c
xenapi: enable user to specify FLAT_NETWORK_BRIDGE

install_os_domU.sh failed, if the FLAT_NETWORK_BRIDGE is found in
localrc. As nova looks up the network by either name-label or bridge
name, it makes sense to enable the user to specify this parameter. As an
example, if the user wants to use name-labels to specify networks, and
those name-labels could be used in domU to create bridges:

VM_BRIDGE_OR_NET_NAME="osvmnet"
FLAT_NETWORK_BRIDGE="osvmnet"

In this case, the domU will know only about a name label, so it could be
decoupled from which xapi bridges used.

This change also adds some fixes (missing double quotes).

Change-Id: I045e367ef441be20c4e8cb8af3c1149392db796b
Make use of STACK_USER instead of relying on USER

Quite easily one ends up calling ./stack.sh in an environment that,
albeit being user "stack" (for example), doesn't quite meet the
expectations of devstack.  The errors that follow can be rather hard to
track down, as the dependency on `USER` is not mentioned.

To remedy this situation, this commit

- uses STACK_USER instead of USER and
- mentions that dependency in the script headers of lib/*

Change-Id: If4cdc39b922ea64b4c0893a0e695ec06349fccc5
Merge "only clone docs from gh-pages branch if they aren't there"
Merge "xenapi: enable user to specify FLAT_NETWORK_BRIDGE"
Merge "Make use of STACK_USER instead of relying on USER"
Merge branch 'master' of github.com:openstack-dev/devstack