Framework: Consolidate Docker commands in site installation

[aduth]

Related: #15159

This pull request seeks to update bin/install-wordpress.sh in an effort to reduce the number of individual docker-compose commands where consolidation is straight-forward.

Each of the affected commands supports multiple arguments:

• mkdir : https://linux.die.net/man/1/mkdir
• chmod : https://linux.die.net/man/1/chmod
• wp plugin install : https://developer.wordpress.org/cli/commands/plugin/install/

Proposed benefit: Anecdotally, each docker-compose command takes upwards of seconds to run, thus encouraging that we reduce the number in the install script. This will shorten build times for Travis and setup times for local end-to-end test runs and environment setups. Additional Docker improvements are proposed at #15159. This will likely have a more minimal impact, but is most trivial to implement.

Testing Instructions:

Verify fresh Docker environments setups for end-to-end tests (and optionally local development).

https://github.com/WordPress/gutenberg/blob/master/docs/contributors/getting-started.md#local-environment

Verify Travis build passes.

[aduth]

Rough measurements:

Control: https://travis-ci.com/WordPress/gutenberg/builds/112450192
Variable: https://travis-ci.com/WordPress/gutenberg/builds/112496088

Task	Before	After
E2E tests (Admin with plugins) (1/4)	11 min 42 sec	11 min 14 sec
E2E tests (Admin with plugins) (2/4)	11 min 31 sec	10 min 48 sec
E2E tests (Admin with plugins) (3/4)	12 min 43 sec	12 min 56 sec
E2E tests (Admin with plugins) (4/4)	11 min 24 sec	10 min 46 sec
E2E tests (Author without plugins) (1/4)	13 min 36 sec	13 min 41 sec
E2E tests (Author without plugins) (2/4)	9 min 52 sec	8 min 39 sec
E2E tests (Author without plugins) (3/4)	12 min 1 sec	10 min 22 sec
E2E tests (Author without plugins) (4/4)	9 min 57 sec	9 min 20 sec

There's some variation within the results, but it generally looks quite positive in the range of 30-45 seconds improvement, which is perhaps more than expected (possibly other factors to consider).

[gziolo]

Yes, it makes sense to consolidate, good idea 👍