Fix lemur-web image build and postgres startup issues

[viranch]

• lemur-web does not build due to missing dependencies
• postgres does not start due to missing POSTGRES_PASSWORD
• lemur-web does not start due to missing failure to connect to the DB

The first issue is fixed by explicitly declaring the dependent packages in the apt-get install command in Dockerfile.

The DB issues are fixed by making the postgres container initialize lemur DB with desired credentials and fixing the wait_db function in lemur-web's entrypoint api-start.sh to use lemur username and password.

[peschmae]

@hosseinsh Any chance you could merge that? Currently the docker-compose config in this repo doesn't work, but these changes would fix that

[hosseinsh]

Thanks @viranch for the PR, and sorry for the long delay, as don't activity develop on this repo. Merging it now.