Provide more build-specific variables

Currently only "custom.p4.revision.number" is available.

It would be great if bamboo's perforce module could additionally provide:

• custom.p4.port - Perforce server as specified in the plan configuration
• custom.p4.client - Perforce clientname (workspace) as specified in the plan configuration
• custom.p4.username - Perforce username as specified in the plan configuration
• custom.p4.revision.previous - The change set number of the previous build (or 0 for initial builds).

We have a number of tests that work directly on the perforce-depot (e.g. doing commit statistics, checks etc.).
Without those variables we would have to specify them a second time (e.g. in the plan's builder arguments).

In addition it would be great to also have global variables for:

• buildForceClean - The flag "Force Clean Build"
• buildTimeStamp - The build's start time
• projectKey - From "Plan Details"
• projectName - From "Plan Details"