About the support level of GitLab and Bitbucket

[sugarnaoming]

What happened:
I believe that GitLab and Bitbucket are not being properly supported.
The reasons are as follows.

• There are some codes in webhooks that can be used only in scm-github.
  e.g. :
  • The part that uses the getCommitSha error.
  • Defining the errors returned by GitLab's getCommitSha()
  • Defining the errors returned by Bitbucket's getCommitSha()
  • Defining the errors returned by GitHub's getCommitSha()
• Functional tests at the time of OSS release do not check the operation of GitLab and Bitbucket.

I would like to discuss how to support these as OSS in the future.

What you expected to happen:
GitLab and Bitbucket will be supported at the same level as Github.

[tk3fftk]

Related: #1652

[tkyi]

Related: #1535