clone - allow alternative repos file to be provided as argument

[tekumara]

eg:

turbolift clone --repos allrepos.txt

this would enable the following workflow:

turbolift clone --repos allrepos.txt

# identify repos that require the change
grep -r ... work/* > repos.txt

# now do the work on just that subset of repos in repos.txt 
turbolift foreach ...

[tekumara]

Closing as this is a duplicate of #74