Refactored platform management Git URL into environment variable

[RobertNorthard]

Refactored platform management Git URL into environment variable ADOP_PLATFORM_MANAGEMENT_GIT_URL

This change was introduced because;

• ADOP Pluggable library supports other SCM providers (Gerrit may not be the default choice)
• enable users to easily use their own platman fork.

Features

• Backwards compatible (defaults to Gerrit).

[subodh-hatkar]

Tested with following steps:

• Spun up ADOP C using "--without-load"
• Changed Load_Platform job configuration to use the fork branch (i.e. the one for this PR)
• Ran Load_Platform
• Verified the SCM repository of Platform Management jobs to have gerrit platform management repo
• Added a global environment variable ADOP_PLATFORM_MANAGEMENT_GIT_URL in Manage Jenkins > Configure System and set it to the my forked adop platform management repo
• Ran Load_Platform again
• Verified the SCM repository of Platform Management jobs to have my forked platform management repo

The repositories are configured as expected if ADOP_PLATFORM_MANAGEMENT_GIT_URL environment variable is set. They fall back to default gerrit repositories if the mentioned variable is not set.

Happy for this PR to be closed.

[RobertNorthard]

@subodh-hatkar thanks for testing.

@nickdgriffin do you have any feedback on this?