Allow choice of GIT protocol used.

From within some corporate http networks, the 'git' protocol cannot be used.
Update the prepare_node and cache_git_repos scripts to default to using the
'git' protocol, but allow customiztion to use the 'http' protocol via command
line argument.

In addition, allow the full base url to be modifed as done in devstack stackrc
to also allow changing repositories.

Change-Id: I875f2e46c466e4e0fb7a5d4dbc54e958d91f9151
Partial-Bug: #1307702
Merge "Allow choice of GIT protocol used."