Add support to discover the blueprint-test test directory from an environment variable

[daniel-cit]

Add support, in the process to auto discover the blueprint-test test directory, to check for the value from an environment variable to discover the integration test directory.

A possible name for the variable could be BPT_TEST_DIR

Code reference:

• cloud-foundation-toolkit/cli/bptest/list.go

  Line 30 in 28bc533

  intTestDir, err := getIntTestDir(intTestDir)

• cloud-foundation-toolkit/cli/bptest/cmd.go

  Line 42 in 28bc533

  intTestDir := flags.testDir

• cloud-foundation-toolkit/cli/bptest/cmd.go

  Line 78 in 28bc533

  intTestDir, err := getIntTestDir(flags.testDir)