use environment variables for GITPROJ and GITBASE
use environment variables for GITPROJ and GITBASE

Companies may have VPN, and in China, there is a firewall named
G.F.W., they both may block some protocols or ports, so if I run
autohelp-wrapper, it will fail like this:

/opt/stack/openstack-doc-tools/autogenerate_config_docs$ ./autohelp-wrapper update ceilometer
Cloning into '/opt/stack/openstack-doc-tools/autogenerate_config_docs/sources/openstack-manuals'...
fatal: unable to connect to git.openstack.org:
git.openstack.org[0: 104.130.246.128]: errno=Connection timed out
git.openstack.org[1: 2001:4800:7819:103:be76:4eff:fe06:63c]: errno=Network is unreachable

Now we have -g option, which can set GITPROJ, but GITBASE still
use git://git.openstack.org/openstack, so it still fail:

/opt/stack/openstack-doc-tools/autogenerate_config_docs$ ./autohelp-wrapper -g https://github.com/openstack update ceilometer

Here I proposal to use environment variable value for GITPROJ and
GITBASE if they exist, so some devs, like me, can get rid of
hacking autohelp-wrapper to work then restore it when commit.

Change-Id: If7dc3ebe398c71d6461a5983d4aab1bc87984697
Closes-Bug: #1574224

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