fix: accept standard browserstack environment variables

[Ankit098]

BROWSERSTACK_USERNAME and BROWSERSTACK_ACCESS_KEY are the env variables specified in docs, check for these. Also accept buildName, local if sent inside 'bstack:options'
Added an issue here: #3149

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[gravityvi]

Thanks @Ankit098 for raising this PR, can you fix the lint issues.

[gravityvi]

Anything mentioned in nightwatch config as ${ENV_VARIABLE} is replaced by process.env.ENV_VARIABLE so we don't need to change the variable names in config.

[beatfactor]

can you format this properly? we don't have lines this long anywhere in the code.

[gravityvi]

done