Do not create PR comment when outside Jenkins environment
Move reset and token arguments to the GitRepoCommand

Both set of options are required to initialize the repository using the
init_main_repo() method. Also remove duplicate option definitions in concrete implementations of this class.
Add tag-release command to recursively tag a repo and its submodules

From an input release number, this command tags the main git repository and
each of the registered submodules. The release number is prefixed by a tag
prefix read from the last tag (using `git describe`).
Add integration test for testing the new tag-release command
Use https URL for sandbox cloning
Replace hard-coded script list by nosetests (in verbose mode)
Add no-ask option to the merge integration tests
Add user.name and user.email to global config
Handle case where git describe fails - return empty tag prefix
Add push argument to tag-release command
Define global github.token to work with Sandbox tests
Set Strict host checking to no for github.com in Travis CI
Pass remote URL with OAuth token when pushing a branch in Sandbox

This commit should allow branches to be pushed to the sandbox fork without
setting SSH keys in the Travis script.
Separate rebase test with or without push to Github
Filter out tests writing back to Github (using Push regexp) from Travis
Split further the rebase tests (--no-push and --no-pr options)
Exclude all rebase tests from Travis build

Rebase tests explicitely require a new PR to be opened which is incompatible
with the read-only token used for testing.
Add check for tag existence before tagging local Git repository

- Raise Stop if tag exists instead of failing during `git tag` call
- Add failing test in TestTagRelease with pre-created tag
Add check for tag validity before tagging local Git repository

- Raise Stop if tag reference name is invalid instead of failing during `git tag` call
- Add failing test in TestTagRelease with invalid tag (ending with '..')
Add check for version format following semver.org in TagRelease

- Add corresponding test with invalid version format
- Rename release argument as version